View source for MI5

{{Short description|British domestic intelligence agency}}
{{other uses|MI-5 (disambiguation)}}
{{distinguish|MI6}}
{{Use British English|date=March 2015}}{{Use dmy dates|date=April 2024}}
{{Infobox government agency
| name            = Security Service (MI5)
| type            = 
| seal            = 
| seal_width      = 
| seal_caption    = 
| seal_alt        = 
| logo            = Mi5 crest and logotype.svg
| logo_width      = 
| logo_caption    = 
| logo_alt        = 
| image           = Thames House and Lambeth Bridge, looking across the river.jpg
| image_size      = 275px
| image_caption   = [[Thames House]], London
| image_alt       = 
| formed          = {{Start date and age|1909|10|7|df=yes}}<br />(as the [[British Secret Service Bureau|Secret Service Bureau]])
| preceding1      = 
| jurisdiction    = [[His Majesty's Government]]
| headquarters    = [[Thames House]], [[London]], [[United Kingdom]]
| coordinates     = {{Coord|51|29|38|N|0|07|32|W|region:GB-WSM_scale:5000|display=title,inline|format=dms|name=Security Service – MI5}}
| motto           = ''Regnum Defende''<br />(Defend the Realm)
| employees       = 5,259<ref name="ISC_2021-2022">[https://isc.independent.gov.uk/wp-content/uploads/2022/12/ISC-Annual-Report-2021%E2%80%932022.pdf Intelligence and Security Committee of Parliament "Annual Report 2021–2022"]</ref>
| budget          = [[Single Intelligence Account]] £3.711 billion (2021–22)<ref name="ISC_2021-2022" />
| minister1_name  = [[Shabana Mahmood]]
| minister1_pfo   = [[Secretary of State for the Home Department|Home Secretary]]
| minister2_name  = 
| minister2_pfo   = 
| chief1_name     = [[Ken McCallum]]
| chief1_position = [[Director General of MI5|Director General]]
| chief2_name     = 
| chief2_position = 
| parent_agency   = 
| child1_agency   = 
| child2_agency   = 
| website         = {{Official URL}}
}}
The '''Security Service''', commonly known as '''MI5''' ([[MI numbers|Military Intelligence, Section 5]]),<ref>{{Cite web |title=What's in a name? |url=https://www.MI5.gov.uk/home/about-us/who-we-are/mi5-history/whats-in-a-name.html |access-date=14 May 2014 |website=www.MI5.gov.uk |publisher=MI5}}</ref> is the United Kingdom's domestic [[counter-intelligence]] and [[security agency]] and is part of its intelligence machinery alongside the [[Secret Intelligence Service]] (MI6), [[Government Communications Headquarters]] (GCHQ), and [[Defence Intelligence]] (DI).  MI5 is directed by the [[Joint Intelligence Committee (United Kingdom)|Joint Intelligence Committee]] (JIC), and the service is bound by the [[Security Service Act 1989]]. The service is directed to protect British parliamentary democracy and economic interests and to counter [[counter-terrorism|terrorism]] and [[counter-espionage|espionage]] within the [[United Kingdom]]. Within the civil service community, the service is colloquially known as ''Box'',<ref>{{cite web| url=https://www.telegraph.co.uk/news/uknews/crime/7969608/Annie-Machon-my-so-called-life-as-a-spy.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.telegraph.co.uk/news/uknews/crime/7969608/Annie-Machon-my-so-called-life-as-a-spy.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=Annie Machon: my so called life as a spy|publisher=The Telegraph|date=29 August 2010|access-date=22 December 2020}}{{cbignore}}</ref> or  ''Box 500'',<ref>{{cite web|url=https://www.independent.co.uk/news/mi5-edges-out-of-the-shadows-42-of-elite-security-service-officers-are-women-terrorists-are-main-target-bugging-of-royal-family-denied-booklet-outlines-organisation-1485397.html|title=MI5 edges out of the shadows: 42% of elite Security Service officers are women – Terrorists are main target – Bugging of Royal Family denied – Booklet outlines organisation|work=The Independent|date=16 July 1993|access-date=22 December 2020}}</ref> after its official wartime address of PO Box 500; its current address is PO Box 3255, [[London]] SW1P 1AE.<ref>{{Cite book|title=The Irish War|first=Tony|last=Geraghty|location=London|publisher=HarperCollins|year=2000|isbn=978-0-00638-674-2}}</ref>

The Security Service is derived from the Secret Service Bureau, founded in 1909. At the start of the [[World War I|First World War]], it was responsible for the arrest of enemy spies, or suspected enemy spies. Throughout the First World War, Germany continually attempted to infiltrate Britain, but MI5 was able to identify most, if not all, of the agents dispatched. During the [[World War II|Second World War]], it developed the [[Double-Cross System]]. This involved attempting to "turn" captured agents wherever possible, and use them to mislead enemy intelligence agencies.

After the war, the service was instrumental in breaking up a large [[Soviet Union|Soviet]] spy ring at the start of the 1970s. It then allegedly became involved in monitoring [[trade union]]s and left-wing politicians. It also assumed responsibility for the investigation of all [[Irish republicanism|Irish republican]] activity within Britain during [[The Troubles]]. Its role was then expanded to countering other forms of terrorism, particularly in more recent years the more widespread threat of [[Islamic extremism]]. In 1996, legislation formalised the extension of the Security Service's statutory remit to include supporting the law enforcement agencies in their work against serious crime.

==Organisation==
=== Structure ===
The Security Service comes under the authority of the [[Home Secretary]] within the [[Cabinet of the United Kingdom|Cabinet]].<ref name=SSAct>{{Cite web|url=https://www.Legislation.gov.uk/ukpga/1989/5/section/1/enacted|title=Security Service Act 1989: The Security Service|website=www.Legislation.gov.uk|publisher=[[HM Government]]|access-date=27 August 2017}}</ref>  The service is headed by a Director General (DG) at the grade of a [[Permanent Secretary]] of the [[British Civil Service|Civil Service]], who is directly supported by an internal security organisation, secretariat, legal advisory branch, and information services branch.  The Deputy Director General is responsible for the operational activity of the service, being responsible for four branches; international counter-terrorism, National Security Advice Centre (counter proliferation and counter espionage), Irish and domestic counter-terrorism, and technical and surveillance operations.<ref>{{Cite web|url=https://www.MI5.gov.uk/people-and-organisation|title=People and organisation|website=www.MI5.gov.uk|publisher=MI5|access-date=21 November 2018}}</ref>

The service is directed for intelligence operational priorities by the Joint Intelligence Committee.<ref name=ISAct>{{Cite web|url=http://www.Legislation.gov.uk/ukpga/1994/13/contents|title=Intelligence Services Act 1994|website=www.Legislation.gov.uk|publisher=[[HM Government]]|access-date=27 August 2017}}</ref> It liaises with SIS, GCHQ, [[Defence Intelligence Staff|DI]], and a number of other bodies within the British government, and industrial base.  It is overseen by the Intelligence and Security Committee of Members of Parliament, who are directly appointed by the [[Prime Minister of the United Kingdom|Prime Minister]], and by the [[Investigatory Powers Commissioner]].<ref>{{Cite web|title=Investigatory Powers Commissioner establishes oversight regime|url=https://www.gov.uk/government/news/investigatory-powers-commissioner-establishes-oversight-regime|access-date=6 January 2021|website=GOV.UK|language=en}}</ref> Judicial oversight of the service's conduct is exercised by the [[Investigatory Powers Tribunal]].<ref>{{Cite web|url=http://www.IPT-UK.com/sections.asp?sectionID=22&type=top|title=What the Tribunal can investigate|website=www.IPT-UK.com|publisher=[[Investigatory Powers Tribunal]]|access-date=6 July 2014}}</ref>

=== Legislation ===
Operations of the service are required to be proportionate, and compliant with British legislation, including the [[Regulation of Investigatory Powers Act 2000]], the [[Investigatory Powers Act 2016]], the [[Data Protection Act 2018]], and various other items of legislation.  Information held by the service is exempt from disclosure under section 23 of the [[Freedom of Information Act 2000]].<ref>{{Cite web|url=http://www.OPSI.gov.uk/acts/acts2000/ukpga_20000036_en_3#pt2-l1g23|title=Freedom of Information Act, section 23|website=www.OPSI.gov.uk|publisher=[[Office of Public Sector Information]]|access-date=3 February 2009}}</ref> The Security Service Act of 1989 establishes several legal key mechanisms aimed at ensuring the accountability and control of the Service.<ref name=":1">{{Cite web|date=13 May 2024 |title=Law, oversight and ethics |url=https://www.mi5.gov.uk/how-we-work/law-oversight-and-ethics|access-date=13 May 2024 |website=MI5}}</ref> Certain provisions from this act were later integrated into the Intelligence Services Act of 1994 with European Commission on Human Rights later having endorsed these mechanisms in multiple applications under the European Convention on Human Rights.<ref>{{Cite web |title=MI5 The Security Service |url=https://cryptome.org/jya/mi5.htm#6 |access-date=2024-05-13 |website=cryptome.org}}</ref> Additionally, the Service is subject to oversight and accountability through various other means. These oversight mechanisms have evolved over time to meet changing governmental requirements, with the Service now being held accountable through a variety of oversight arrangements.<ref name=":1" /><ref>{{Cite web |title=MI5 The Security Service |url=https://cryptome.org/jya/mi5.htm#6 |access-date=13 May 2024|website=cryptome.org}}</ref>
=== Oversight ===
All employees of the service are bound by the [[Official Secrets Act 1989|Official Secrets Act]].<ref>{{Cite book|last1=Leach|first1=Robert|last2=Coxall|first2=Bill|last3=Robins|first3=Lynton|date=17 August 2011|title=British Politics|url=https://books.google.com/books?id=l6McBQAAQBAJ&q=MI5+officers+bound+by+the++Official+Secrets+Act&pg=PA341|publisher=Palgrave Macmillan|isbn=978-0-230-34422-8|page=341|access-date=11 July 2015}}</ref>  In certain circumstances, officers handling agents or informers may authorise them to carry out activity which would otherwise be criminal within the United Kingdom.<ref name=Guardian-20180302>{{Cite news|last=Grierson|first=Jamie|date=2 March 2018|url=https://www.TheGuardian.com/uk-news/2018/mar/02/mi5-agents-are-allowed-to-commit-in-uk-government-reveals|title=MI5 agents can commit crime in UK, government reveals|website=[[The Guardian]]|access-date=3 March 2018}}</ref> The Security Service "is authorised to investigate any person or movement that might threaten the...security" of the United Kingdom.<ref name=":0">{{Cite web |date=11 April 2023 |title=MI5 |url=https://www.britannica.com/topic/MI5 |access-date=9 May 2023 |publisher=[[Encyclopedia Britannica]] |language=en}}</ref> The current Director General is [[Ken McCallum]], who succeeded [[Andrew Parker, Baron Parker of Minsmere|Andrew Parker]] in April 2020.<ref name="HomeOffice">{{Cite web|title=Appointment of the new Director General of the Security Service|url=https://www.GOV.uk/government/news/appointment-of-the-new-director-general-of-the-security-service|website=www.GOV.uk|publisher=[[Home Office]]|date=28 March 2013|access-date=20 August 2013}}</ref>

=== Centenary and awards ===
The service marked its centenary in 2009 by publishing an official history titled ''[[The Defence of the Realm|The Defence of the Realm: The Authorized History of MI5]]'', written by [[Christopher Andrew (historian)|Christopher Andrew]], Professor of Modern and Contemporary History at [[University of Cambridge|Cambridge University]].<ref>{{Cite web|url=https://www.MI5.gov.uk/home/mi5-history/mi5---the-authorised-centenary-history.html|title=MI5 – The authorised centenary history|website=www.MI5.gov.uk|publisher=MI5|archive-url=https://web.archive.org/web/20130930065302/https://www.mi5.gov.uk/home/mi5-history/mi5---the-authorised-centenary-history.html|archive-date=30 September 2013|access-date=8 June 2013|url-status=live}}</ref> Members of the Security Service are recognised annually by [[King Charles III]] at the [[Prince of Wales's Intelligence Community Awards]] at [[St James's Palace]] or [[Clarence House]] alongside members of the [[MI6|Secret Intelligence Service]] (MI6), and [[GCHQ]].<ref name="Mayer2016">{{cite book|last=Mayer|first=Catherine|title=Charles: the Heart of a King|publisher=[[Ebury Publishing]]|year=2016|isbn=978-0-7535-5595-8|url=https://books.google.com/books?id=ZpnACwAAQBAJ&pg=PA175|access-date=24 October 2021|page=175}}</ref> Awards and citations are given to teams within the agencies as well as individuals.<ref name="Mayer2016" />

==History==
===Early years===
The Security Service is derived from the '''Secret Service Bureau''', founded in 1909, and concentrating originally on the activities of the [[German Empire|Imperial German]] government, as a joint initiative of the [[British Admiralty|Admiralty]] and the [[War Office]].  The Bureau was initially split into naval and army sections which, over time, specialised respectively in foreign target [[espionage]] and internal [[counter-espionage]] activities. The former  specialisation was a result of a growing interest at the Admiralty, at the time, in intelligence regarding the fleet of the [[Imperial German Navy]].  This division was formalised, as separate home and foreign sections, prior to the beginning of the [[First World War]].  Following a number of administrative changes, the home section became known as [[Directorate of Military Intelligence (United Kingdom)|Directorate of Military Intelligence]], Section 5 and the abbreviation MI5, the name by which it is still known in popular culture.<ref>{{Cite web|url=http://www.SIS.gov.uk/output/Page470.html|title=SIS Records — War Office Military Intelligence (MI) Sections in the First World War|website=www.SIS.gov.uk|publisher=[[Secret Intelligence Service|SIS]]|url-status=dead|archive-url=https://web.archive.org/web/20060820014944/http://www.sis.gov.uk/output/Page470.html|archive-date=20 August 2006|access-date=21 November 2018}}</ref> (The foreign/naval section of the Secret Service Bureau was to become the basis of the later Secret Intelligence Service, or [[MI6]]).<ref>{{cite web |last= Whitehead|first=Jennifer|url=https://www.sis.gov.uk/our-mission.html|title=Our mission | publisher =SIS|date=15 July 2016 | access-date=25 August 2017}}</ref>

The founding head of the Army section was [[Vernon Kell]] of the [[South Staffordshire Regiment]], who remained in that role until the early part of the [[Second World War]].  Its role was originally quite restricted, as the section existed solely to ensure national security through counter-espionage.  With a small staff, and working in conjunction with the [[Special Branch (Metropolitan Police)|Special Branch]] of the [[Metropolitan Police]], the service was responsible for overall direction and the identification of foreign agents, while Special Branch provided the manpower for the investigation of their affairs, arrest and interrogation.<ref>{{Cite news|url=https://www.Telegraph.co.uk/news/uknews/1497979/End-for-Special-Branch-after-122-years.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.Telegraph.co.uk/news/uknews/1497979/End-for-Special-Branch-after-122-years.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=End for Special Branch after 122 years|website=www.Telegraph.co.uk|date=9 September 2005|publisher=The Telegraph|access-date=21 November 2018}}{{cbignore}}</ref>

On the day after the declaration of the First World War, the Home Secretary, [[Reginald McKenna]], announced that "within the last twenty-four hours no fewer than twenty-one spies, or suspected spies, have been arrested in various places all over the country, chiefly in important military or naval centres, some of them long known to the authorities to be spies".<ref>{{Cite Hansard|speaker=[[Reginald McKenna]]|position=Home Secretary|date=5 August 1914|title=Aliens Restriction Bill|url=https://api.parliament.uk/historic-hansard/commons/1914/aug/05/aliens-restriction-bill|house=[[House of Commons]]|column=1985}}</ref> These arrests have provoked recent historical controversy.  According to the official history of MI5, the actual number of agents identified was 22, and Kell had started sending out letters to local police forces on 29 July, giving them advance warning of arrests to be made as soon as war was declared.  [[Portsmouth Constabulary]] jumped the gun and arrested one on 3 August, and not all of the 22 were in custody by the time that McKenna made his speech, but the official history regards the incident as a devastating blow to [[Imperial Germany]], which deprived them of their entire spy ring, and specifically upset the Kaiser.<ref>{{Cite book|first=Christopher|last=Andrew|title=The Defence of the Realm: The Authorized History of MI5|publisher=Allen Lane|year=2009|pages=49–52}}</ref>

In 2006, his article 'Entering the Lists' was published in the journal ''Intelligence and National Security'', outlining the products of his research into recently opened files.<ref>{{Cite journal|first=Nicholas|last=Hiley|title=Entering the Lists: MI5's great spy round-up of August 1914|journal=Intelligence and National Security|volume=21|issue=1|year=2006|pages=46–76|doi=10.1080/02684520600568303|s2cid=154556503}}</ref>  Hiley was sent an advance copy of the official history, and objected to the retelling of the story.  He later wrote another article, 'Re-entering the Lists', which asserted that the list of those arrested published in the official history<ref>{{Cite book|first=Christopher|last=Andrew|title=The Defence of the Realm: The Authorized History of MI5|publisher=Allen Lane|year=2009|pages=873–875}}</ref> was concocted from later case histories.<ref>{{Cite journal|first=Nicholas|last=Hiley|title=Re-entering the Lists: MI5's Authorized History and the August 1914 Arrests|journal=Intelligence and National Security|volume=25|issue=4|year=2010|pages=415–452|doi=10.1080/02684527.2010.537022|s2cid=153404992}}</ref>

===Inter-war period===
MI5 proved consistently successful throughout the rest of the 1910s and 1920s in its core counter-espionage role.  Throughout the First World War, Germany continually attempted to infiltrate Britain, but MI5 was able to identify most, if not all, of the agents dispatched. MI5 used a method that depended on strict control of entry and exit to the country and, crucially, large-scale inspection of mail. In post-war years, attention turned to attempts by the [[Soviet Union]] and the [[Comintern]] to surreptitiously support revolutionary activities within Britain. MI5's expertise, combined with the early incompetence of the Soviets, meant the bureau was successful in correctly identifying and closely monitoring these activities.<ref>{{Cite news|url= https://www.Telegraph.co.uk/history/10755313/How-MI5-combated-Communist-attempts-to-take-over-the-scouts.html|archive-url= https://web.archive.org/web/20140410114821/http://www.telegraph.co.uk/history/10755313/How-MI5-combated-Communist-attempts-to-take-over-the-scouts.html|url-status= dead|archive-date= 10 April 2014|title= How MI5 combated Communist attempts to take over the scouts|website= www.Telegraph.co.uk|publisher= The Telegraph|date= 10 April 2014|access-date=21 November 2018}}</ref>

In the meantime, MI5's role had grown substantially. Due to the spy hysteria, MI5 had formed with far more resources than it actually needed to track down German spies.  As is common within governmental bureaucracies, this caused the service to expand its role to use its spare resources. MI5 acquired many additional responsibilities during the war.  Most significantly, its strict counter-espionage role blurred considerably. It acquired a much more political role, involving the surveillance not merely of foreign agents, but also of [[Pacifism|pacifist]] and anti-[[conscription]] organisations, and of [[Trade unionism|organised labour]].  This was justified by citing the common belief that foreign influence was at the root of these organisations. Thus, by the end of the First World War, MI5 was a fully-fledged investigating force (although it never had powers of arrest), in addition to being a counter-espionage agency. The expansion of this role continued after a brief post-war power struggle with the head of the [[Special Branch]], Sir [[Basil Thomson]].<ref>{{Cite web|url= http://www.Spartacus.schoolnet.co.uk/SSthomson.htm|title=Basil Thomson|website= www.Spartacus.schoolnet.co.uk|publisher=[[Spartacus Educational]]|access-date=1 July 2012|url-status= dead |archive-url= https://web.archive.org/web/20120520021351/http://www.spartacus.schoolnet.co.uk/SSthomson.htm|archive-date= 20 May 2012}}</ref>

After the First World War, budget-conscious politicians regarded Kell's department as unnecessary.  In 1919, MI5's budget was slashed from £100,000 to just £35,000, and its establishment from over 800 officers to a mere 12. At the same time, Sir [[Basil Thomson]] of Special Branch was appointed Director of Home Intelligence, in supreme command of all domestic counter-insurgency and counter-intelligence investigations.  Consequently, as official MI5 historian [[Christopher Andrew (historian)|Christopher Andrew]] has noted in his official history ''Defence of the Realm'' (2010), MI5 had no clearly defined role in the [[Anglo-Irish War]] of 1919–1921. To further worsen the situation, several of Kell's officers defected to Thomson's new agency, the Home Intelligence Directorate.  MI5 therefore undertook no tangible intelligence operations of consequence during the [[Irish War of Independence]].  MI5 did undertake the training of [[British Army]] case-officers from the Department of Military Intelligence (DMI), for the Army's so-called "Silent Section", otherwise known as M04(x). Quickly trained by MI5 veterans at [[Cavalry Barracks, Hounslow|Hounslow Barracks]], outside London, these freshly-minted M04(x) Army case-officers were deployed to [[Dublin]] beginning in the spring of 1919.  Over time, 175 officers were trained and dispatched to Ireland. In Ireland, they came under the command of General [[Cecil Romer]] and his Deputy, Lieutenant Colonel Stephen Searle Hill-Dillon.<ref>{{Cite web|url= http://www.BloodySunday.co.uk/castle-intelligence/hill-dillon/hill-dillon.html |title= Stephen Searle Hill-Dillon|website= www.BloodySunday.co.uk|publisher= Bloody Sunday|access-date= 21 November 2018}}</ref>

In April 1919, Colonel Walter Wilson of the Department of Military Intelligence arrived in Dublin to take over the day-to-day management of these 175 Army intelligence-officers, and the unit was designated as the "Dublin District Special Branch" (DMI/MO4(x)/DDSB), because it operated exclusively within the confines of the Army's Dublin Military District. Royal Marine Colonel [[Hugh Montgomery (Royal Marines officer)|Hugh Montgomery]] of the Department of Naval Intelligence, was also seconded to Romer's intelligence staff at this time.<ref name="Hittle">{{Cite book|first= J. B. E.|last= Hittle|author-link= J.B.E. Hittle|title= Michael Collins and the Anglo-Irish War: Britain's Failed Counterinsurgency|location= Washington, D.C.|publisher= Potomac Books|year= 2011|isbn= 978-1-59797-535-3}}</ref> British Army after-action reports and contemporary accounts indicate that M04(x)/DDSB was considered by some a highly amateurish outfit. Serious cover constraints, coupled with alcohol abuse and social fraternisation with local prostitutes would prove the downfall of several of these amateur sleuths.<ref name="Hittle" /> Despite these failings, it was not MI5, but one of Basil Thomson's agents, John Charles Byrnes, a [[double agent]] within the IRA, who identified [[Michael Collins (Irish leader)|Michael Collins]], and came close to arranging his capture. The IRA identified Byrnes as a British spy and murdered him in March 1920.<ref>{{Cite web |url= http://www.BloodySunday.co.uk/castle-intelligence/thomson/byrnes/charles-byrnes.html|title=John Charles Byrnes or Jack Jameson|website=www.BloodySunday.co.uk}}</ref>

The intelligence staff of Michael Collins [[Irish Republican Army]] penetrated the unit.<ref>{{Cite book|first= T. Ryle|last=Dwyer|year=2005|title=The Squad and the Intelligence Operations of Michael Collins|location= Cork |publisher=[[Mercier Press]]|isbn=978-1-85635-469-1}}</ref>  Using DMP detectives [[Ned Broy]] and [[David Nelligan]], Michael Collins was able to learn the names and lodgings of the M04(x) agents, referred to by IRA operatives as "The Cairo Gang". On "[[Bloody Sunday (1920)|Bloody Sunday]]", 21 November 1920, Collins ordered his counter-intelligence unit, [[The Squad (IRA unit)|The Squad]], to assassinate 25 M04(x) agents, several British courts-martial officers, at least one agent reporting to Basil Thomson, and several intelligence officers attached to the [[Royal Irish Constabulary Auxiliary Division]], at their lodgings throughout Dublin.<ref name="Hittle" /> That afternoon, a mixed force of the British Army, the [[Royal Irish Constabulary]], and the [[Black and Tans]] retaliated by indiscriminately shooting dead 14 civilians at a [[Gaelic Football]] match at [[Croke Park]].<ref>{{Cite news|url= https://www.BBC.co.uk/news/world-europe-13434653|title= Croke Park: Queen in emotionally charged visit|publisher=[[BBC News]]|date=18 May 2011|access-date=1 July 2012}}</ref> 

The net impact of Collins's strike of Bloody Sunday, 21 November 1920, was relatively negligible, even though the IRA had not gone up against MI5 professionals, but instead only against a quickly trained outfit of amateur army "D-Listers".<ref name="Hittle" /> Although the shooting of 14 British officers had the desired effect on British morale, other aspects were botched by the IRA. Three of Collins's men were apprehended after engaging in a shoot-out on the street, and at least two of the wounded British officers had no connection whatsoever to British intelligence. Moreover, with MO4(x) having fielded a total of 175 agents of the DDSB, Collins's operation only temporarily slowed British momentum.  Within days, the remaining 160-odd M04(x) agents were re-established in secure quarters inside solidly loyalist hotels in Dublin, from where they continued to pursue Collins and the IRA relentlessly right up until the truce of July 1921.<ref name="Hittle" />

In December 1920, the entire DDSB was transferred from [[British Army]] command to civil command under Deputy Police Commissioner General Ormonde Winter, and thereafter was known as "D Branch" within Dublin Castle. By January 1921, the highly experienced [[MI6]] operative David Boyle arrived at Dublin Castle to take over the day-to-day management of D Branch.  The unit's former commander, Colonel Wilson, resigned in protest against having had his command taken from him. D Branch thrived under Boyle's leadership.<ref name="Hittle" /> 

In 1921, Sir [[Warren Fisher (civil servant)|Warren Fisher]], the government inspector-general for civil-service affairs, conducted a thorough review of the operations and expenditures of Basil Thomson's Home Intelligence Directorate. He issued a scathing report, accusing Thomson of wasting both money and resources, and conducting redundant as well as ineffectual operations. Shortly thereafter, in a private meeting with Prime Minister [[David Lloyd George]], Sir Basil Thomson was sacked, and the Home Intelligence Directorate was formally abolished.  With Thomson out of the way, [[Special Branch]] was returned to the command of the Commissioner of The Criminal Investigation Division at [[Scotland Yard]]. Only then was Vernon Kell able once again to rebuild MI5 and re-establish it in its former place as Britain's chief domestic spy agency.<ref name=Hittle/>

MI5 operated in Italy during inter-war period, and helped [[Benito Mussolini]] get his start in politics with a £100 weekly wage.<ref name=Guardian2009-10-13>{{Cite news|first=Tom|last=Kington|date=13 October 2009|title= Recruited by MI5: the name's Mussolini. Benito Mussolini |url=https://www.TheGuardian.com/world/2009/oct/13/benito-mussolini-recruited-mi5-italy|website=[[The Guardian]]|location=London|access-date=14 October 2009}}</ref> MI5's efficiency in counter-espionage declined from the 1930s. It was to some extent a victim of its own success as it was unable to break the ways of thinking it had evolved in the 1910s and 1920s. In particular, it was unable to adjust to the new methods of the Soviet intelligence services, the [[NKVD|People's Commissariat for Internal Affairs]] (NKVD) and [[GRU (G.U.)|Main Intelligence Directorate]] (GRU). It continued to think in terms of agents who would attempt to gather information simply through observation or bribery, or to agitate within labour organisations and the armed services, while posing as ordinary citizens. The NKVD, meanwhile, had evolved more sophisticated methods; it began to recruit agents from within the [[upper classes]] (most notably from [[University of Cambridge|Cambridge University]]), whom it regarded as a long-term investment. Such NKVD agents succeeded in gaining positions within the government, and, in [[Kim Philby |Kim Philby's]] case, within British intelligence itself, from where they were able to provide the NKVD with sensitive information.  The most successful of these agents; [[Kim Philby|Harold 'Kim' Philby]], [[Donald Duart Maclean|Donald Maclean]], [[Guy Burgess]], [[Anthony Blunt]], and [[John Cairncross]]; went undetected until after the [[World War II|Second World War]], and became known as the [[Cambridge Five]].<ref name="CambridgeBBC">{{Cite web|url= https://www.BBC.co.uk/history/historic_figures/spies_cambridge.shtml|title=Historic Figures: The Cambridge Spies |publisher=[[BBC|British Broadcasting Corporation]]|access-date=1 July 2012}}</ref>

===Second World War===
One of the earliest actions of [[Winston Churchill]] on coming to power in early 1940 was to sack the agency's long-term head, [[Vernon Kell]].  He was replaced initially by the ineffective [[Oswald Allen Harker]], as Acting Director General.  Harker in turn was quickly replaced by [[David Petrie]], a [[Secret Intelligence Service]] (SIS) man, with Harker remaining as his deputy.  With the ending of the [[Battle of Britain]], and the abandonment of invasion plans (correctly reported by both SIS and the [[Bletchley Park]] [[Ultra (cryptography)|Ultra]] project), the spy scare eased, and the internment policy was gradually reversed.  This eased pressure on MI5, and allowed it to concentrate on its major wartime success, the so-called [[Double Cross System|'double-cross' system]].<ref name="Masterman">{{Cite book|last=Masterman|first=John Cecil|author-link=John Cecil Masterman|year=1972|orig-year=1945|title=The Double-Cross System in the War of 1939 to 1945|publisher=[[Australian National University Press]]|isbn=978-0-7081-0459-0}}</ref>  This was a system based on an internal memo drafted by an MI5 officer in 1936, which criticised the long-standing policy of arresting and sending to trial all enemy agents discovered by MI5.  Several had offered to defect to Britain when captured; before 1939, such requests were invariably turned down.  The memo advocated attempting to 'turn' captured agents wherever possible, and use them to mislead enemy intelligence agencies.  This suggestion was turned into a massive and well-tuned system of deception during the Second World War.<ref name="Masterman" />

Beginning with the capture of an agent named [[Arthur Owens]], codenamed 'Snow', MI5 began to offer enemy agents the chance to avoid prosecution (and thus the possibility of the death penalty) if they would work as British [[double-agent]]s.  Agents who agreed to this were supervised by MI5 in transmitting bogus 'intelligence' back to the German secret service, the [[Abwehr]].  This necessitated a large-scale organisational effort, since the information had to appear valuable but actually be misleading.  A high-level committee, the Wireless Board, was formed to provide this information.  The day-to-day operation was delegated to a sub-committee, the Twenty Committee (so called because the Roman numerals for twenty, XX, form a double cross).<ref name=Masterman/> The system was extraordinarily successful.  A post-war analysis of German intelligence records found that of the 115 or so agents targeted against Britain during the war, all but one (who committed suicide) had been successfully identified and caught, with several 'turned' to become double agents.  The system played a major part in the massive campaign of deception which preceded the [[D-Day]] landings, designed to give the Germans a false impression of the location and timings of the landings (see [[Operation Fortitude]]).<ref name=Masterman/>

While the double-cross work dealt with enemy agents sent into Britain, a smaller-scale operation run by [[Victor Rothschild, 3rd Baron Rothschild|Victor Rothschild]] targeted British citizens who wanted to help Germany.  The '[[Fifth Column (intelligence operation)|Fifth Column]]' operation saw an MI5 officer, [[Eric Roberts (spy)|Eric Roberts]], masquerade as the [[Gestapo|Gestapo's]] man in London, encouraging Nazi sympathisers to pass him information about people who would be willing to help Germany in the event of invasion.  When his recruits began bringing in intelligence, he promised to pass that on to Berlin. The operation was deeply controversial within MI5, with opponents arguing that it amounted to entrapment.  By the end of the war, Roberts had identified around 500 people.  But MI5 decided not to prosecute, and instead covered the work up, even giving some of Roberts' recruits Nazi medals. They were never told the truth.<ref>{{Cite book|last=Hutton|first=Robert|date=2019|title=Agent Jack : the true story of MI5's secret Nazi hunter|publisher=Weidenfeld & Nicolson|isbn=978-1474605137|oclc=994362312}}</ref>

All foreigners entering the country were processed at the [[London Reception Centre]] (LRC) at the [[Royal Victoria Patriotic Building]], which was operated by MI5 subsection B1D; 30,000 were inspected at LRC. Captured enemy agents were taken to [[Camp 020]], [[Latchmere House]], for interrogation.  This was commanded by Colonel Robin Stephens. There was a reserve camp, Camp 020R, at [[Huntercombe (HM Prison)|Huntercombe]], which was used mainly for long term detention of prisoners.<ref>{{Cite book|first=Oliver|last=Hoare|year=2000|title=Camp 020: MI5 and the Nazi Spies — the official history of MI5's wartime interrogation centre|publisher=[[Public Record Office]]|isbn=978-1-903365-08-3}}</ref> It is believed that two MI5 officers participated in "a gentle interrogation" given to the senior Nazi [[Heinrich Himmler]] after his arrest at a military checkpoint in the northern German village of Bremervörde in May 1945.  Himmler subsequently killed himself during a medical examination by a British officer by means of a cyanide capsule that he had concealed in his mouth.  One of the MI5 officers, [[Sidney Henry Noakes]] of the [[Intelligence Corps (United Kingdom)|Intelligence Corps]], was subsequently given permission to keep Himmler's [[Suspenders|brace]]s and the forged identity document that had led to his arrest.<ref name="BBCNoakes">{{Cite web|url=https://www.BBC.co.uk/news/uk-52755565|title=Heinrich Himmler: how a fake stamp led to the Nazi SS leader's capture|website=www.BBC.co.uk|date=23 May 2020|publisher=[[BBC News]]|access-date=25 May 2020}}</ref><ref name="Mil-Intel">{{Cite web|title=Sidney Noakes Intelligence Corps officer with MI5: Himmler's false identity document|url=https://www.MilitaryIntelligenceMuseum.org/sidney-noakes|website=www.MilitaryIntelligenceMuseum.org|publisher=Military Intelligence Museum|archive-url=https://web.archive.org/web/20200525084642/https://www.militaryintelligencemuseum.org/sidney-noakes|access-date=25 May 2020|archive-date=25 May 2020|url-status=live}}</ref>

===Post-Second World War===
The Prime Minister's personal responsibility for the service was delegated to the [[Secretary of State for the Home Department|Home Secretary]] [[David Maxwell Fyfe, 1st Earl of Kilmuir|David Maxwell-Fyfe]] in 1952, with a directive issued by the Home Secretary setting out the role and objectives of the Director General.  The service was subsequently placed on a statutory basis in 1989 with the introduction of the Security Service Act.  This was the first government acknowledgement of the existence of the service.<ref>{{Cite web|url=http://www.Archive.Official-Documents.co.uk/document/cm47/4779/4779.htm|title=Security Service Act 1989|website=www.Archive.Official-Documents.co.uk|date=4 July 2000|access-date=1 July 2012}}</ref> The post-war period was a difficult time for the service, with a significant change in the threat as the [[Cold War]] began, being challenged by an extremely active [[KGB]], and increasing incidence of the Northern Ireland conflict, and international [[terrorism]].  Whilst little has yet been released regarding the successes of the service, there have been a number of intelligence failures which have created embarrassment for both the service and the government.  For instance, in 1983, one of its officers, [[Michael Bettaney]], was caught trying to sell information to the KGB.  He was subsequently convicted of espionage.<ref name="SpyTelegraph">{{Cite news|first=David|last=Harrison|date=11 November 2007|url=https://www.Telegraph.co.uk/news/uknews/1569004/Cold-War-rivals-play-at-spy-game.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.Telegraph.co.uk/news/uknews/1569004/Cold-War-rivals-play-at-spy-game.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=Cold War rivals play at spy game|publisher=[[The Daily Telegraph]]|access-date=1 July 2012}}{{cbignore}}</ref>

Following the [[Michael Bettaney]] case, [[Philip Woodfield]] was appointed as a staff counsellor for the security and intelligence services.  His role was to be available to be consulted by any member or former member of the security and intelligence services who had "anxieties relating to the work of his or her service"<ref>{{Cite Hansard|speaker=Malcolm Sinclair, The Earl of Caithness|speaker-link=Malcolm Sinclair, 20th Earl of Caithness|position=Minister of State, Home Office|date=30 November 1987|title=Security Services Ombudsman: Access|url=https://api.parliament.uk/historic-hansard/lords/1987/nov/30/security-services-ombudsman-access|house=[[House of Lords]]|column=811}}</ref> that it had not been possible to allay through the ordinary processes of management-staff relations, including proposals for publications.<ref>{{Cite Hansard|speaker=[[John Patten, Baron Patten|John Patten]]|position=[[Under-Secretary of State for the Home Department|Minister for Home Affairs]]|date=21 December 1988|title=Official Secrets Bill|url=https://api.parliament.uk/historic-hansard/commons/1988/dec/21/official-secrets-bill|house=[[House of Commons]]|column=538}}</ref> The service was instrumental in breaking up a large [[Soviet Union|Soviet]] spy ring at the start of the 1970s, with 105 Soviet embassy staff known or suspected to be involved in intelligence activities being expelled from the country in 1971.<ref name="SpyTelegraph" />

One episode involving MI5 and the [[BBC]] came to light in the mid-1980s. MI5 officer [[Ronnie Stonham]] had an office in the BBC, and took part in vetting procedures.<ref>{{Cite book|first1=Mark|last1=Hollingsworth|first2=Richard|last2=Norton-Taylor|year=1988|title=Blacklist: The Inside Story of Political Vetting|location=London|publisher=[[Hogarth Press]]|page=104|isbn=978-0-70120-811-0}}</ref> Controversy arose when it was alleged that the service was monitoring [[trade union]]s and left-wing politicians. A file was kept on Labour Prime Minister [[Harold Wilson]] from 1945, when he became a [[Member of Parliament (United Kingdom)|Member of Parliament]] (MP), although the agency's official historian, [[Christopher Andrew (historian)|Christopher Andrew]] maintains that his fears of MI5 conspiracies and bugging were unfounded.<ref>{{Cite news|url=https://news.bbc.co.uk/2/hi/uk_news/8288247.stm|title=MI5 kept file on former PM Wilson|website=News.BBC.co.uk|publisher=[[BBC News]]|date=3 October 2009}}</ref> As Home Secretary, the [[Labour Party (UK)|Labour]] MP [[Jack Straw]] discovered the existence of his own file dating from his days as a [[student radical]].<ref>{{Cite news|first=Sarah|last=Schaefer|date=22 January 1999|url=https://www.Independent.co.uk/news/parliament--politics-straw-will-not-see-his-mi5-file-1075428.html|title=Parliament & Politics: Straw will not see his MI5 file|website=www.Independent.co.uk|publisher=[[The Independent]]|access-date=1 July 2012}}</ref>

One of the most significant and far-reaching failures was an inability to conclusively detect and apprehend the '[[Cambridge Five]]' spy ring, which had formed in the inter-war years, and achieved great success in penetrating the government, and the intelligence agencies themselves.<ref name=CambridgeBBC/>  Related to this failure were suggestions of a high-level penetration within the service, [[Peter Wright (MI5 officer)|Peter Wright]] (especially in his controversial book ''[[Spycatcher]]'') and others believing that evidence implicated the former Director General, [[Roger Hollis]], or his deputy [[Graham Russell Mitchell|Graham Mitchell]]. The [[Burke Trend, Baron Trend|Trend]] inquiry of 1974 found the case unproven of that accusation, and that view was later supported by the former KGB officer [[Oleg Gordievsky]].<ref>{{Cite news|first=James|last=Bamford|date=18 November 1990|url=https://www.NYTimes.com/1990/11/18/books/gordievsky-s-people.html|title=Gordievsky's people|website=[[The New York Times]]|access-date=1 July 2012}}</ref>  Another spy ring, the [[Portland spy ring]], exposed after a tip-off by Soviet defector [[Michael Goleniewski]], led to an extensive MI5 surveillance operation.<ref>{{Cite news|first1=Jason|last1=Lewis|first2=Jonathan|last2=Wynne-Jones|date=18 June 2011|url=https://www.Telegraph.co.uk/news/religion/8584157/MI5-labelled-the-Archbishop-of-Canterbury-a-subversive-over-anti-Thatcher-campaigns.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.Telegraph.co.uk/news/religion/8584157/MI5-labelled-the-Archbishop-of-Canterbury-a-subversive-over-anti-Thatcher-campaigns.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=MI5 labelled the Archbishop of Canterbury a subversive over anti-Thatcher campaigns|publisher=[[The Daily Telegraph]]|access-date=1 July 2012}}{{cbignore}}</ref>

In 1991, MI5 revealed its head publicly for the first time and declassified some information, "such as the number of its employees and its organizational structure."<ref name=":0" /> There have been strong accusations levelled against MI5 for having failed in its obligation to provide care for former police agents who had infiltrated the [[Provisional Irish Republican Army|Provisional IRA]] during [[the Troubles]]. The two most notable of the agents, [[Martin McGartland]] and [[Raymond Gilmour]], went on to reside in England using false identities, and in 2012, launched test cases against the agency. Both men claimed to journalist Liam Clarke in the ''Belfast Telegraph'' that they were abandoned by MI5 and were "left high and dry despite severe health problems as a result of their work and lavish promises of life-time care from their former Intelligence bosses". Both men suffer from [[post-traumatic stress disorder]] (PTSD).<ref name="BelfastTelegraph">{{Cite news|first=Liam|last=Clarke|date=14 September 2012|url=http://www.BelfastTelegraph.co.uk/news/local-national/northern-ireland/two-exspies-target-mi5-in-landmark-legal-battle-over-payouts-16210559.html|title=Two ex-spies target MI6 in landmark legal battle over payouts|website=www.BelfastTelegraph.co.uk|publisher=[[Belfast Telegraph]]|access-date=7 January 2013}}</ref>

Following the [[United States invasion of Afghanistan]], on 9 January 2002, the first MI5 staff arrived at [[Bagram Airfield|Bagram]]. On 12 January 2002, following a report by an [[MI6]] officer that a detainee appeared to have been mistreated before, an MI6 officer was sent instructions that were copied to all MI5 and MI6 staff in Afghanistan about how to deal with concerns over mistreatment, referring to signs of abuse: 'Given that they are not within our custody or control, the law does not require you to intervene to protect this'. It went on to say that the Americans had to understand that the UK did not condone such mistreatment, and that a complaint should be made to a senior US official if there was any coercion by the US in conjunction with an MI6 interview.<ref>{{Cite book|last=Corera|first=Gordon|author-link=Gordon Corera|year=2012|title=MI6: Life and Death in the British Secret Service|publisher=[[W&N]]|isbn=978-0753828335|page=339}}</ref>

===Counter-terrorism===
[[File:Thames house exterior.jpg|thumb|Part of [[Thames House]]]]
The end of the [[Cold War]] resulted in a change in emphasis for the operations of the service, assuming responsibility for the investigation of all [[Irish republicanism|Irish republican]] activity within Britain,<ref>{{Cite web|url=https://www.NYTimes.com/2012/05/20/world/europe/7-suspected-of-ties-to-ira-faction-are-charged-with-terrorism.html|title=7 tied to faction of the IRA face terrorism charges|date=19 May 2012|website=[[The New York Times]]|access-date=20 November 2011}}</ref> and increasing the effort countering other forms of terrorism, particularly in more recent years the more widespread threat of [[Islamic extremism]].<ref>{{Cite web|first=Alasdair|last=Palmer|date=14 May 2006|url=https://www.Telegraph.co.uk/news/uknews/1518320/MI5-mission-impossible.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.Telegraph.co.uk/news/uknews/1518320/MI5-mission-impossible.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=MI5 mission: impossible|publisher=[[The Daily Telegraph]]|access-date=1 July 2012}}{{cbignore}}</ref>

Whilst British security forces have provided support in countering the activities of both [[Irish republicanism|Irish republican]] and [[Ulster loyalism|Ulster loyalist]] [[paramilitary]] groups in [[Northern Ireland]] since the early 1970s, republicans have often accused these forces of [[collusion]] with loyalists. In 2006, an Irish government committee inquiry claimed to have found nine instances of collusion between British security forces and loyalist paramilitaries during the 1970s.<ref>{{Cite news|url=http://www.IrishTimes.com/newspaper/breaking/2006/1129/breaking86.htm|title=Barron finds British collusion in attacks |url-access=subscription |date=29 November 2006|publisher=[[The Irish Times]] |url-status=dead |archive-url= https://archive.today/20130127075813/http://www.irishtimes.com/newspaper/breaking/2006/1129/breaking86.html |archive-date= 27 January 2013 }}</ref><ref>{{Cite web|url=http://Burnsmoley.com/pages/collusion/reports/Kays_Tavern.pdf|title=Final report on the Report of the Independent Commission of Inquiry into the Bombing of Kay's Tavern, Dundalk|date=November 2006|website=Burns Moley|publisher=[[Houses of the Oireachtas]]|access-date=20 November 2011|url-status=usurped|archive-url=https://web.archive.org/web/20081026014447/http://burnsmoley.com/pages/collusion/reports/Kays_Tavern.pdf|archive-date=26 October 2008}}</ref> In 2012, a document-based review by the British barrister [[Desmond de Silva (barrister)|Sir Desmond de Silva]] into the 1989 murder of Belfast solicitor [[Pat Finucane]] found that MI5 had spread propaganda against Finucane in the years prior to his death and took no steps to protect him from loyalist paramilitaries.<ref name=PatFinucaneReview>{{Cite web|url=http://www.PatFinucaneReview.org/report/volume01/chapter011/|title=Volume 1 chapter 11: The flow of information from members of the security forces to the UDA |publisher=Pat Finucane Review|archive-url=https://web.archive.org/web/20121216022945/http://www.patfinucanereview.org/report/volume01/chapter011/|archive-date=16 December 2012|url-status=dead|access-date=28 November 2017}}</ref> The review disclosed that MI5 assessments of [[Ulster Defence Association]] intelligence consistently noted that the majority came from MI5 sources, with an assessment in 1985 finding 85% of the intelligence originating from MI5.<ref name=PatFinucaneReview/>  [[British Prime Minister]] [[David Cameron]], in response to the review, apologised on behalf of the UK government to Finucane's family.<ref>{{Cite web|title=Pat Finucane murder: 'Shocking state collusion', says PM|url=https://www.BBC.co.uk/news/uk-northern-ireland-20662412|publisher=[[BBC News]]|date=12 December 2012|access-date=16 July 2017}}</ref>

On 10 October 2007, the lead responsibility for national security intelligence in Northern Ireland returned to the Security Service from the [[Police Service of Northern Ireland]] (PSNI), that had been devolved in 1976 to the [[Royal Ulster Constabulary]] (RUC) during [[Ulsterisation]].<ref>{{Cite web|title=MI5 in Northern Ireland|url=https://www.MI5.gov.uk/mi5-in-northern-ireland|website=www.MI5.gov.uk|publisher=Security Service MI5|access-date=15 July 2017}}</ref><ref>{{Cite web|title=Transfer of national security lead to the Security Service|url=http://www.PSNI.Police.uk/index/departments/crime/transfer_of_national_security_lead_to_the_security_service_page.htm|website=www.PSNI.Police.uk|publisher=[[Police Service of Northern Ireland]]|archive-url=https://web.archive.org/web/20080608095622/http://www.psni.police.uk/index/departments/crime/transfer_of_national_security_lead_to_the_security_service_page.htm|archive-date=8 June 2008|url-status=dead|access-date=28 November 2017}}</ref>  During April 2010, the [[Real IRA]] detonated a 120&nbsp;lb [[car bomb]] outside Palace Barracks in [[County Down]], which is the headquarters of MI5 in Northern Ireland and also home to the 2nd Battalion [[The Mercian Regiment]].<ref>{{Cite news|url=https://news.bbc.co.uk/2/hi/8670037.stm|title=Man arrested over Palace Barracks bomb released|website=News.BBC.co.uk|date=9 May 2010|publisher=[[BBC News]]|access-date=1 July 2012}}</ref>

MI5 is understood to have a close working relationship with the [[Republic of Ireland]]'s [[Special Detective Unit]] (SDU), the counter-terrorism and counter-intelligence section of the [[Garda Síochána]] (national police), particularly with regards to threats from [[dissident republican]] terrorism and [[Islamic terrorism]].<ref>{{Cite news|last=McDonald|first=Henry|date=2 March 2008|title=MI5 targets Ireland's al-Qaeda cells|url=https://www.TheGuardian.com/world/2008/mar/02/alqaida.ireland|website=[[The Guardian]]|access-date=5 June 2014}}</ref>
[[File:MI5-GCHQ Memorial.jpg|alt=Memorial in the cloister of Westminster Abbey, London|thumb|Recognising service at home and abroad protecting the United Kingdom]]
Executive liaison groups enable MI5 to safely share secret, sensitive, and often raw intelligence with the police, on which decisions can be made about how best to gather evidence and prosecute suspects in the courts.  Each organisation works in partnership throughout the investigation, but MI5 retain the lead for collecting, assessing and exploiting intelligence.  The police take lead responsibility for gathering evidence, obtaining arrests, and preventing risks to the public.<ref>{{Cite book|first=Kim|last=Howells|author-link=Kim Howells|date=May 2009|url=http://www.CabinetOffice.gov.uk/media/210852/20090519_77review.pdf|title=Could 7/7 have been prevented? Review of the Intelligence on the London terrorist attacks on 7 July 2005|publisher=UK Cabinet Office, Intelligence and Security Committee|location=London|archive-url=https://swap.stanford.edu/20120503112509/http://www.cabinetoffice.gov.uk/media/210852/20090519_77review.pdf|archive-date=3 May 2012|access-date=25 May 2009|url-status=dead}}</ref>

===Serious crime===
In 1996, legislation formalised the extension of the Security Service's statutory remit to include supporting the law enforcement agencies in their work against serious crime.<ref name=febrefa>{{Cite Hansard|speaker=[[Baroness Blatch]]|position=Minister of State, Home Office|date=10 June 1996|house=[[House of Commons]]|title=Security Service Bill|url=https://Publications.Parliament.uk/pa/ld199596/ldhansrd/vo960610/text/60610-07.htm|column_start=1502|column_end=1503}}</ref>  Tasking was reactive, acting at the request of law enforcement bodies such as the [[National Criminal Intelligence Service]] (NCIS), for whom MI5 officers performed electronic surveillance and eavesdropping duties during [[Clerkenwell crime syndicate|Operation Trinity]].<ref name=febrefa/>  This role has subsequently been passed to the [[Serious Organised Crime Agency]] (SOCA) and then the [[National Crime Agency]] (NCA).<ref>{{Cite web|url=http://www.nationalcrimeagency.gov.uk/about-us|title=National Crime Agency – About us|website=www.NationalCrimeAgency.gov.uk|publisher=[[National Crime Agency]]|access-date=21 November 2018|archive-date=12 July 2020|archive-url=https://web.archive.org/web/20200712212024/https://www.nationalcrimeagency.gov.uk/404|url-status=dead}}</ref>

===Surveillance===
In 2001, after the [[September 11 attacks]] in the US, MI5 started collecting bulk telephone communications data under a little understood general power of the [[Telecommunications Act 1984]] (instead of the [[Regulation of Investigatory Powers Act 2000]] which would have brought independent oversight and regulation). This was kept secret until announced by the [[Home Secretary]] in 2015.<ref name=BBC-20151105>{{Cite web|first=Gordon|last=Corera|author-link=Gordon Corera|date=5 November 2015|url=https://www.BBC.co.uk/news/uk-politics-34731735|title=How and why MI5 kept phone data spy programme secret|publisher=[[BBC News]]|access-date=9 November 2015}}</ref><ref name=Telegraph-20151104>{{Cite web|first=Tom|last=Whitehead|date=4 November 2015|url=https://www.Telegraph.co.uk/news/uknews/terrorism-in-the-uk/11976008/MI5-and-GCHQ-secretly-bulk-collecting-British-publics-phone-and-email-records-for-years-Theresa-May-reveals.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.Telegraph.co.uk/news/uknews/terrorism-in-the-uk/11976008/MI5-and-GCHQ-secretly-bulk-collecting-British-publics-phone-and-email-records-for-years-Theresa-May-reveals.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|title=MI5 and GCHQ secretly bulk collecting British public's phone and email records for years, Theresa May reveals|publisher=[[The Daily Telegraph]]|access-date=9 November 2015}}{{cbignore}}</ref><ref>{{Cite web|url=https://www.TheRegister.co.uk/2015/11/09/hawktalk_wip/|title=Here's the little-known legal loophole that permitted mass surveillance in the UK|website=www.TheRegister.co.uk|publisher=[[The Register]]|date=9 November 2015|access-date=9 November 2015}}</ref> This power was replaced by the [[Investigatory Powers Act 2016]]<ref>{{Cite web|year=2016|title=Investigatory Powers Act 2016|url=https://www.legislation.gov.uk/ukpga/2016/25/schedule/10/paragraph/99|website=Legislation.gov.uk}}</ref> which introduced new surveillance powers overseen by the Investigatory Powers Commission (IPC) it introduces.<ref>{{Cite web|title=Investigatory Powers Bill|url=https://researchbriefings.files.parliament.uk/documents/CBP-7518/CBP-7518.pdf |archive-url=https://web.archive.org/web/20160323143708/http://researchbriefings.files.parliament.uk/documents/CBP-7518/CBP-7518.pdf |archive-date=23 March 2016 |url-status=live|website=House of Commons Library}}</ref><ref>{{Cite web|title=30 October 2015 Investigatory Powers Commission factsheet|url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/473744/Factsheet-Investigatory_Powers_Commission.pdf |archive-url=https://web.archive.org/web/20190725233305/https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/473744/Factsheet-Investigatory_Powers_Commission.pdf |archive-date=25 July 2019 |url-status=live}}</ref>

In July 2006, parliamentarian [[Norman Baker]] accused the British Government of "hoarding information about people who pose no danger to this country", after it emerged that MI5 holds secret files on 272,000 individuals, equivalent to one in 160 adults.  It had previously been revealed that a '[[traffic light]]' system operates:<ref>{{Cite Hansard|speaker=[[Jack Straw]]|position=[[Home Secretary]]|date=25 February 1998|house=[[House of Commons]]|title=Security Service files|url=https://Publications.Parliament.uk/pa/cm199798/cmhansrd/vo980225/debtext/80225-16.htm#80225-16_spnew1|column_start=346|column_end=348}}</ref><ref>{{Cite Hansard|house=[[House of Commons]]|title=MI5 files|url=https://Publications.Parliament.uk/pa/cm200506/cmhansrd/vo060605/text/60605w0055.htm#0606078000026|date=5 June 2006|column=278W}}</ref>
*Green: active; about 10% of files
*Amber: enquiries prohibited, further information may be added; about 46% of files
*Red: enquiries prohibited, substantial information may not be added; about 44% of files.

===Participation of MI5 officers in criminal activity===
In March 2018, the government acknowledged that MI5 officers are allowed to authorise agents to commit criminal activity in the UK.  Maya Foa, the director of [[Reprieve (organisation)|Reprieve]], said: "After a seven-month legal battle, the prime minister has finally been forced to publish her secret order, but we are a long way from having transparency.  The public and parliament are still being denied the guidance that says when British spies can commit criminal offences, and how far they can go. Authorised criminality is the most intrusive power a state can wield.  Theresa May must publish this guidance without delay".<ref name=Guardian-20180302/>

In November 2019, four [[human rights organisation]]s claimed that the UK government has a policy dating from the 1990s to allow MI5 officers to authorise agents or informers to participate in crime, and to immunise them against prosecution for criminal actions.  The organisations said the policy allowed MI5 officers to authorise agents and informers to participate in criminal activities that protected national security or the economic well-being of the UK.  The organisations took the UK government to the [[Investigatory Powers Tribunal]], seeking to have it declare the policy illegal, and to issue an injunction against further 'unlawful conduct'.<ref>{{Cite web|last=Osborne|first=Samuel|date=8 November 2019|title=MI5 licensed informants to commit murder, kidnap and torture for decades, court hears|url=https://www.Independent.co.uk/news/uk/crime/mi5-informants-commit-crimes-murder-kidnap-torture-security-service-a9187976.html|website=www.Independent.co.uk|publisher=[[The Independent]]|access-date=9 November 2019}}</ref>  In December 2019, the tribunal dismissed the request of the human rights organisations in a 3-to-2 decision.  The potential criminal activities include murder, kidnap, and torture, according to a [[Bloomberg L.P.|Bloomberg]] report.<ref>{{Cite news|last=Browning|first=Jonathan|date=20 December 2019|title=Court rules British MI5 agents can murder, kidnap and torture|url=https://www.Bloomberg.com/news/articles/2019-12-20/licensed-to-kill-court-rules-mi5-can-murder-kidnap-torture|newspaper=Bloomberg.com|publisher=[[Bloomberg L.P.|Bloomberg]]|access-date=2 January 2020}}</ref>

===Allegations of collusion in torture===
In October 2020, [[Rangzieb Ahmed]] brought a civil claim against MI5, alleging that Pakistan's [[Inter-Services Intelligence]] agency had arrested him in 2006, and that MI5 had colluded in torture by submitting questions which were put to him under torture in Pakistan.<ref>{{Cite web|url=https://www.TheGuardian.com/uk-news/2020/oct/27/mi5-colluded-in-pakistans-torture-of-british-terrorist-court-hears?|title=MI5 colluded in Pakistan's torture of British terrorist, court hears|website=[[The Guardian]]|date=27 October 2020}}</ref> This claim was rejected by the High Court on 16 December 2020.<ref>{{cite web|url=https://www.judiciary.uk/judgments/rangzieb-ahmed-v-director-general-of-security-service-and-others/|title=Rangzieb Ahmed -v- Director General of Security Service and others|publisher=High Court|date=16 December 2020|access-date=31 March 2021}}</ref>

=== Domestic abuse scandal ===
In May 2022, the BBC reported on an unnamed MI5 agent, 'X', domestically abusing his partner, despite the British government's attempts to block this.<ref>{{Cite news |date=2022-05-19 |title=MI5 agent used secret status to terrorise girlfriend |url=https://www.bbc.com/news/uk-61508520 |access-date=2025-06-04 |work=BBC News |language=en-GB}}</ref> BBC journalists subsequently discovered that MI5 lied to courts, suggesting that the organisation was unaware of X's behaviour, including his far-right extremist views.<ref>{{Cite web |date=2025-02-12 |title=How I exposed MI5's lie about its violent abusive agent |url=https://www.bbc.com/news/articles/cx25m231n8xo |access-date=2025-06-04 |website=BBC News |language=en-GB}}</ref> An ongoing High Court case began in June 2025, with MI5 accepting that X's status should no longer be protected, after previously [[Glomar response|neither confirming nor denying]] his role as an agent.<ref>{{Cite web |title=MI5 accepts neo-Nazi agent's spy status should no longer be protected |url=https://www.bbc.co.uk/news/live/cn8z78vjkn1t |access-date=2025-06-04 |website=BBC News |language=en-GB}}</ref>

==Buildings==
MI5 was based at Watergate House in the [[Strand, London|Strand]] from 1912 until 1916, when it moved to larger facilities at 16 [[Charles II Street|Charles Street]] for the remaining years of the [[World War I|First World War]].<ref>{{Cite book|last=Andrew|first=Christopher|title=The Defence of the Realm: The Authorized History of MI5|page=[https://Archive.org/details/defenceofrealmau0000andr/page/85 85]|publisher=Allen Lane|year=2009|isbn=978-0-713-99885-6|url=https://Archive.org/details/defenceofrealmau0000andr/page/85}}</ref>  After the First World War, it relocated to smaller premises at 73–75 [[Queen's Gate]] in 1919,<ref>{{Cite book|last=Andrew|first=Christopher|title=The Defence of the Realm: The Authorized History of MI5|page=[https://Archive.org/details/defenceofrealmau0000andr/page/117 117]|publisher=Allen Lane|year=2009|isbn=978-0-713-99885-6|url=https://Archive.org/details/defenceofrealmau0000andr/page/117}}</ref> and then moved to 35 [[Cromwell Road]] in 1929, before transferring to the top floor of the South Block of [[Thames House]] on [[Millbank]] in 1934.<ref>{{Cite book|last=Andrew|first=Christopher|title=The Defence of the Realm: The Authorized History of MI5|page=[https://Archive.org/details/defenceofrealmau0000andr/page/134 134]|publisher=Allen Lane|year=2009|isbn=978-0-713-99885-6|url=https://Archive.org/details/defenceofrealmau0000andr/page/134}}</ref>  The Service spent the first year of the [[World War II|Second World War]] at [[HM Prison Wormwood Scrubs|Wormwood Scrubs]], before moving to [[Blenheim Palace]], Oxfordshire, in 1940.<ref>{{Cite book|last=Andrew|first=Christopher|title=The Defence of the Realm: The Authorized History of MI5|page=[https://Archive.org/details/defenceofrealmau0000andr/page/217 217]|publisher=Allen Lane|year=2009|isbn=978-0-713-99885-6|url=https://Archive.org/details/defenceofrealmau0000andr/page/217}}</ref>  After the Second World War, MI5 was based at [[Leconfield House]] (1945–1976), and [[140 Gower Street]] (1976–1994, since demolished),<ref>{{Cite web|url=http://www.GeoCities.ws/foxeye121/curzon.htm|title=MI5 (The Security Service)|website=www.GeoCities.ws|publisher=The Secret Architecture of London|access-date=18 February 2017}}</ref> before returning to Thames House in 1994.<ref>{{Cite book|last=Sheldon|first=Robert|author-link=Robert Sheldon, Baron Sheldon|date=June 1993|title=Thames House and Vauxhall Cross|url=http://www.NAO.org.uk/wp-content/uploads/2000/02/9900236.pdf |archive-url=https://web.archive.org/web/20140222043827/http://www.nao.org.uk/wp-content/uploads/2000/02/9900236.pdf |archive-date=22 February 2014 |url-status=live|publisher=[[National Audit Office (United Kingdom)|National Audit Office]]|location=London|isbn=978-0-10556-669-4|page=43|access-date=7 July 2013}}</ref>

The national headquarters at Thames House draws together personnel from a number of locations into a single HQ facility: Thames House also houses the [[Joint Terrorism Analysis Centre]] (JTAC), a subordinate organisation to the Security Service; prior to March 2013, Thames House additionally housed the [[Northern Ireland Office]] (NIO).  The service has offices across the United Kingdom, including a HQ in [[Northern Ireland]].<ref>{{Cite web|title=Intelligence, counter-terrorism, and trust|url=https://www.MI5.gov.uk/home/about-us/who-we-are/staff-and-management/director-general/speeches-by-the-director-general/intelligence-counter-terrorism-and-trust.html|website=www.MI5.gov.uk|publisher=MI5|date=5 November 2007|access-date=8 January 2015}}</ref> Details of a northern operations centre in [[Greater Manchester]] were revealed by the firm who built it.<ref>{{Cite web|first=David|last=Leppard|date=14 June 2009|url=http://www.TimesOnline.co.uk/tol/news/politics/article6493658.ece|title=Oops! Building firm blurts out secrets of hush-hush MI5 HQ|website=www.TimesOnline.co.uk|publisher=[[The Sunday Times]]|access-date=28 December 2011|archive-url=https://web.archive.org/web/20100615225627/http://www.timesonline.co.uk/tol/news/politics/article6493658.ece|archive-date=15 June 2010|url-status=dead}}</ref>

==Directors General of the Security Service==
[[File:Stella Rimington.JPG|thumb|[[Stella Rimington|Dame Stella Rimington]], the first female [[Director General of MI5]]]]
{{Main|Director General of MI5}}
*1909–1940: [[Vernon Kell|Sir Vernon Kell]] (born 1873 – d. 1942)
*1940–1941: [[Oswald Allen Harker]] (born 1886 – d. 1968)
*1941–1946: [[David Petrie|Sir David Petrie]] (born 1879 – d. 1961)
*1946–1953: [[Percy Sillitoe|Sir Percy Sillitoe]] (born 1888 – d. 1962)
*1953–1956: [[Dick White]] (born 1906 – d. 1993)
*1956–1965: [[Roger Hollis]] (born 1905 – d. 1973)
*1965–1972: [[Martin Furnival Jones]] (born 1912 – d. 1997)
*1972–1979: [[Michael Hanley]] (born 1918 – d. 2001)
*1979–1981: [[Howard Smith (diplomat)|Howard Smith]] (born 1919 – d. 1996)
*1981–1985: [[John Jones (MI5 officer)|John Jones]] (born 1923 – d. 1998)
*1985–1988: [[Antony Duff]] (born 1920 – d. 2000)
*1988–1992: [[Patrick Walker (MI5 officer)|Patrick Walker]] (born 1932 – d. 2021)
*1992–1996: [[Stella Rimington]] (born 1935 – d. 2025)
*1996–2002: [[Stephen Lander]] (born 1947)
*2002–2007: [[Eliza Manningham-Buller]] (born 1948)
*2007–2013: [[Jonathan Evans, Baron Evans of Weardale|Jonathan Evans]] (born 1958)
*2013–2020: [[Andrew Parker, Baron Parker of Minsmere|Andrew Parker]] (born 1962)
* 2020–present: [[Ken McCallum]] (born 1974)

==Past names==
Although the Service is commonly referred to as 'MI5', this was its official name for only thirteen years (1916–1929). However, it is still used as a sub-title on the various pages of the official Security Service website, as well as in their web address (https://www.MI5.gov.uk).
*October 1909: founded as the '''Home Section of the Secret Service Bureau''';
*April 1914: became a sub-section of the War Office '''Directorate of Military Operations, section 5''' (MO5) — MO5(g);
*September 1916: became '''Military Intelligence section 5''' — MI5;
*1929: renamed the '''Defence Security Service''';
*1931: renamed the '''Security Service'''.

===Cover name===
MI5 has previously used '''Government Communications Planning Directorate''' (GCPD) as a cover name when sponsoring research related to drone usage in charging recording devices.<ref>{{Cite news |title=MI5 involvement in drone project revealed in paperwork slip-up |last=Sabbagh |first=Dan |work=The Guardian |date=6 March 2021 |url= https://www.theguardian.com/uk-news/2021/mar/06/mi5-involvement-in-drone-project-revealed-in-paperwork-slip-up}}</ref>

==Coat of arms==
{{incomplete section|date=July 2025|small=no}}
{{Infobox COA wide
| image             =
| year_adopted      = 1981
| motto             = '''REGNUM DEFENDE'''
| notes             = <ref>{{Cite web|url=https://www.mi5.gov.uk/faq |access-date=29 December 2022 |title='MI5 FAQ'}}</ref>
}}

==See also==
*[[British intelligence agencies]]
*[[Annie Machon]] – MI5 whistleblower
*[[Delores Kane]] – MI5 whistleblower
*[[Club de Berne]] – a European intelligence sharing forum
*[[Counter Terrorism Policing]]
*[[Counter Terrorism Command]] – of London's Metropolitan Police Service
*[[Joint Terrorism Analysis Centre]] (JTAC)
*[[National Protective Security Authority]] – child agency of MI5 which provides security advice
*[[Pat Finucane]] – Irish lawyer murdered by Ulster Defence Association members with the collusion of the security service

==References==
{{Reflist|32em}}

==Further reading==
*{{Cite book|last1=Aldrich|first1=R. J.|first2=R.|last2=Cormac|year=2016|title=The Black Door: Spies, Secret Intelligence and British Prime Ministers|publisher=Collins|isbn=978-0-00-755544-4}}
*{{Cite book|last=Andrew|first=Christopher|author-link=Christopher Andrew (historian)|year=2009|title=The Defence of the Realm: The Authorized History of MI5|publisher=Allen Lane|isbn=978-1-84614-284-0|title-link=The Defence of the Realm: The Authorized History of MI5}}  Published as {{Cite book|place=USA|date=November 2009|publisher=Knopf|title=Defend the Realm: The Authorized History of MI5|isbn=978-0-307-26363-6|url-access=registration|url=https://Archive.org/details/defendrealmautho00andr}}.
*{{Cite book|last=Curry|first=John|title=The Security Service, 1908–1945|publisher=[[Public Record Office]]|year=1999|isbn=978-1-873162-79-8}}
*{{Cite book|last1=Hennessey|first1=Thomas|first2=Claire|last2=Thomas|year=2009|title=Spooks: The Unofficial History of MI5 from the First Atom Spy to 7/7, 1945–2009|publisher=Amberley|isbn=978-1-84868-079-1}}
*{{Cite book|last1=Hennessey|first1=Thomas|first2=Claire|last2=Thomas|year=2010|title=Spooks: The Unofficial History of MI5 from Agent ZIGZAG to the D-Day Deception, 1939–45|publisher=Amberley Publishing|isbn=978-1-4456-0184-7}}
*{{Cite book|last=Machon|first=A.|year=2005|title=Spies, Lies and Whistleblowers: MI5, MI6 and the Shayler Affair|publisher=The Book Guild|isbn=978-1-85776-952-4}}
*{{Cite book|last=Milne|first=Seumas|author-link=Seumas Milne|year=2014|title=The Enemy Within: The Secret War Against the Miners|publisher=Verso Books|isbn=978-1-78168-342-2|title-link=The Enemy Within: The Secret War Against the Miners}}
*{{Cite book|last=Murphy|first=Christopher J.|year=2006|title=Security and Special Operations: SOE and MI5 During the Second World War|location=New York|publisher=Palgrave Macmillan|isbn=978-0-230-00241-8}}
*{{Cite book|last=Pincher|first=Chapman|year=2011|title=Treachery Betrayals, Blunders and Cover Ups: Six Decades of Espionage|publisher=Mainstream Publishing|isbn=978-1-78057-540-7}}
*{{Cite book|last=Quinlan|first=Kevin|year=2014|title=The Secret War Between the Wars: MI5 in the 1920s and the 1930s|publisher=Bowyer|isbn=978-1-84383-938-5}}
*{{Cite book|last=Rimington|first=Stella|author-link=Stella Rimington|year=2001|url=http://www.abebooks.com/servlet/BookDetailsPL?bi=2226267574&searchurl=kn%3Drimington%2520memoirs%26sts%3Dt|title=Open Secret: The Autobiography of the Former Director-General of MI5|publisher=Hutchinson|isbn=978-0-09943-672-0}}
*{{Cite book|last=Thomas|first=Martin|year=2008|title=Empires of Intelligence: Security Services and Colonial Disorder after 1914|publisher=University of California Press|isbn=978-0-52025-117-5}}
*{{Cite book|last=Thurlow|first=R.|year=1994|title=The Secret State: British Internal Security in the Twentieth Century|publisher=Blackwell|isbn=978-0-631-16066-3}}
*{{Cite book|last=West|first=Nigel|author-link=Nigel West|year=1981|title=A British Security Service Operations, 1939–1945|publisher=Bodley Head|isbn=978-0-370-30324-6}}
*{{Cite book|last=West|first=Nigel|year=1982|title=A Matter of Trust: MI5, 1945–72|publisher=Weidenfeld & Nicolson|isbn=978-0-340-33781-3}}
*{{Cite book|last=West|first=Nigel|year=2012|title=Mask: MI5's Penetration of the Communist Party of Great Britain|publisher=Routledge|isbn=978-0-415-35145-4}}

==External links==
{{Commons category|Security Service of the United Kingdom}}
* {{Official website}}
*[https://www.NationalArchives.gov.uk/documentsonline/securityservice.asp Records of the Security Service]—from [[The National Archives (United Kingdom)|The National Archives]]
*[https://www.CabinetOffice.gov.uk/security_and_intelligence/community.aspx/ UK Intelligence community online]—from the [[Cabinet Office]]

{{UK Intelligence Agencies}}
{{National intelligence agencies}}
{{National intelligence agencies in Europe}}
{{Five Eyes}}
{{Authority control}}

[[Category:MI5| ]]
[[Category:1909 establishments in the United Kingdom]]
[[Category:Domestic intelligence agencies]]
[[Category:British intelligence agencies]]
[[Category:Buildings and structures in the City of Westminster]]
[[Category:Intelligence services of World War II]]
[[Category:Military units and formations of the United Kingdom in World War II]]