MI1

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.

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.