Hut 3 was a section of the Government Code and Cypher School (GC&CS) at Bletchley Park during World War II. It retained the name for its functions when it moved into Block D. It produced military intelligence codenamed Ultra from the decrypts of Enigma, Tunny and multiple other sources. Hut 3 thus became an intelligence agency in its own right, providing information of great strategic value, but rarely of operational use. Group Captain Eric Malcolm Jones led this activity from 1943 and after the war became deputy director, and in 1952 director of GCHQ. In July 1945, General Dwight D. Eisenhower Supreme Commander of Allied forces wrote to Sir Stewart Menzies, Chief of the British Secret Intelligence Service (MI6) saying inter alia:"The intelligence that has emanated from you before and during this campaign has been of priceless value to me. It has simplified my task as commander enormously. It has saved thousands of British and American lives and, in no small way, has contributed to the speed with which the enemy was routed and eventually forced to surrender."

Bletchley Park is an English country house and estate in Bletchley, Milton Keynes (Buckinghamshire), that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following 1883 for the financier and politician Herbert Leon in the Victorian Gothic, Tudor and Dutch Baroque styles, on the site of older buildings of the same name.