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{{Short description|Department of the British Directorate of Military Intelligence}}
{{Use dmy dates|date=December 2014}}
{{Use British English|date=December 2014}}
{{otheruses|MI-1 (disambiguation)}}

'''MI1''' or British ''Military Intelligence, Section 1'' was a department of the British [[Directorate of Military Intelligence (United Kingdom)|Directorate of Military Intelligence]], part of  the [[War Office]]. It was set up during [[World War I cryptography|World War I]]. It contained "C&C", which was responsible for code breaking.<ref>Gannon, 2011</ref>

Its subsections in World War I were;
* MI1a: Distribution of reports, intelligence records.
* MI1b: Interception and [[cryptanalysis]].
* MI1c: [[Secret Intelligence Service|The Secret Service/SIS]].
* MI1d: Communications security.
* MI1e: Wireless telegraphy.
* MI1f: Personnel and finance.
* MI1g: Security, deception and counter intelligence.

From 1915, MI1(b) was headed by [[Malcolm Vivian Hay]]. [[Oliver Strachey]] was in MI1 during World War I. He transferred to the [[Government Code and Cypher School]] and served there during World War II. [[John Tiltman]] was seconded to MI1 shortly before it merged with [[Room 40]]. 

In 1919 MI1b and the Royal Navy's (NID25) "[[Room 40]]" were closed down and merged into the inter-service Government Code and Cypher School,<ref>Gannon, 2011</ref><ref> {{Harvnb|Erskine|Smith|2011|p=14}} </ref> which subsequently developed into the [[Government Communications Headquarters]] (GCHQ) at Cheltenham.

==Notes==
{{reflist}}

==References==
* [https://www.mi5.gov.uk/faq#collapseOne_195 What happened to MI1 - MI4?]
* {{Citation | editor-last = Erskine | editor-first = Ralph | editor2-last = Smith | editor2-first = Michael | editor-link = Michael Smith (newspaper reporter) | title = The Bletchley Park Codebreakers | publisher = Biteback Publishing Ltd | year = 2011 | isbn = 978-1-84954-078-0}} Updated and extended version of ''Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer'' Bantam Press 2001
* Gannon, Paul, ''Inside Room 40: The Codebreakers of World War I'', Ian Allan Publishing, 2011, {{ISBN|978-0-7110-3408-2}}

{{UK_Intelligence Agencies}}

{{DEFAULTSORT:Mi1}}
[[Category:Cryptography organizations]]
[[Category:Defunct United Kingdom intelligence agencies]]
[[Category:1910s establishments in the United Kingdom]]
[[Category:Military units and formations disestablished in 1919]]
[[Category:United Kingdom in World War I]]
[[Category:Military communications of the United Kingdom]]