MI1 or British Military Intelligence, Section 1 was a department of the British Directorate of Military Intelligence, part of the War Office.
[1] Its subsections in World War I were; From 1915, MI1(b) was headed by Malcolm Vivian Hay.
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.
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.