An improved version of 'Fat Man' was developed, and on 26 February 1952, Prime Minister Winston Churchill announced that the United Kingdom also had an atomic bomb and a successful test took place on 3 October 1952. At first these were free-fall bombs, intended for use by the V Force of jet bombers. A Vickers Valiant dropped the first UK nuclear weapon on 11 October 1956 at Maralinga, South Australia. Later came a missile, Blue Steel, intended for carriage by the V Force bombers, and then the Blue Streak medium-range ballistic missile (later canceled). Anglo-American cooperation on nuclear weapons was restored by the 1958 US-UK Mutual Defence Agreement. As a result of this and the Polaris Sales Agreement, the United Kingdom has bought United States designs for submarine missiles and fitted its own warheads. It retains full independent control over the use of the missiles. It no longer possesses any free-fall bombs.