On 1 November 1919 the Army signals intelligence department at M11b and the Naval signals intelligence from the Admiralty’s Room 40 combined to become the Government Code and Cypher School, known as GC&CS. Led by Commander Alastair Denniston, and based at Watergate House in The Strand in London, GC&CS comprised just fifty personnel.
GC&CS relocated to Bletchley Park for the duration of WW2. Once the war was over the newly re-named GCHQ moved to Eastcote. Between 1950 and 1954 the organisation relocated once more to Cheltenham which remains its primary base. On 1 November 2019 GCHQ celebrates its 100th anniversary.