Which British intelligence agency employed Alan Turing during World War II?

The British intelligence agency that employed Alan Turing during World War II was the Government Code and Cypher School, commonly abbreviated GC&CS. It was Britain's codebreaking and signals-intelligence organisation, based largely at Bletchley Park during the war.

Turing began working part-time for GC&CS in September 1938 and reported to Bletchley Park soon after Britain declared war in September 1939. There, he worked mainly on German Enigma ciphers, first alongside Dilly Knox and later with the wider Hut 8 team focused on naval Enigma traffic.

GC&CS was not MI5, MI6, or the Special Operations Executive. MI5 handled domestic security, MI6 conducted overseas intelligence operations, and the Special Operations Executive supported sabotage and resistance movements. GC&CS concentrated on codes, ciphers, and signals intelligence. In 1946, it was renamed the Government Communications Headquarters, or GCHQ.