During the early part of the Second World War , Britain had a nuclear weapons project , codenamed Tube Alloys . At the Quebec Conference in August 1943 , the Prime Minister , Winston Churchill and the President of the United States , Franklin Roosevelt , signed the Quebec Agreement , which merged Tube Alloys with the American Manhattan Project to create a combined British , American and Canadian project . The British government trusted that the United States would continue to share nuclear technology , which it regarded as a joint discovery , after the war , but the United States Atomic Energy Act of 1946 ( McMahon Act ) ended technical cooperation . Fearing a resurgence of United States isolationism , and Britain losing its great power status , the British government restarted its own development effort , now codenamed High Explosive Research . The first British atomic bomb was tested in Operation Hurricane on 3 October 1952 . The subsequent British development of the hydrogen bomb , and a favourable international relations climate created by the sputnik crisis , led to the McMahon Act being amended in 1958 , resulting in the restoration of the nuclear Special Relationship in the form of the 1958 US – UK Mutual Defence Agreement , which allowed Britain to acquire nuclear weapons systems from the United States
Operation Hurricane
Operation Hurricane
Nassau Agreement