MI1 or BritishMilitary Intelligence, Section 1 was a department of the BritishDirectorate of Military Intelligence, part of theWar Office. It was set up duringWorld War I. It contained "C&C", which was responsible for code breaking.[1]
Its subsections in World War I were;
From 1915, MI1(b) was headed byMalcolm Vivian Hay.Oliver Strachey was in MI1 during World War I. He transferred to theGovernment Code and Cypher School and served there during World War II.John Tiltman was seconded to MI1 shortly before it merged withRoom 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,[2][3] which subsequently developed into theGovernment Communications Headquarters (GCHQ) at Cheltenham.