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.[1]

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),[2][3] which subsequently developed into the Government Communications Headquarters (GCHQ) at Cheltenham.

Oliver Strachey was in MI1 during World War I. He transferred to GC&CS and served there during World War II. John Tiltman was seconded to MI1 shortly before it merged with Room 40.


  1. Gannon, 2011
  2. Gannon, 2011
  3. Erskine & Smith 2011, p. 14


  • 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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