MI1 or British Military Intelligence, Section 1 was a department of the British Directorate of Military Intelligence, part of the War Office. It was set up during World War I. It contained "C&C", which was responsible for code breaking.
Its subsections in World War I were:
- MI1a: Distribution of reports, intelligence records.
- MI1b: Interception and cryptanalysis.
- MI1c: The Secret Service/SIS.
- MI1d: Communications security.
- MI1e: Wireless telegraphy.
- MI1f: Personnel and finance.
- MI1g: Security, deception and counter intelligence.
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 (GC&CS), which subsequently developed into the Government Communications Headquarters (GCHQ) at Cheltenham.
- What happened to MI1 - MI4?
- Erskine, Ralph; Smith, Michael, eds. (2011), The Bletchley Park Codebreakers, Biteback Publishing Ltd, 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
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