Tube Alloys was the research and development programme authorised by the United Kingdom, with participation from Canada, to develop nuclear weapons during the Second World War. Starting before the Manhattan Project in the United States, the British efforts were kept classified, and as such had to be referred to by code even within the highest circles of government.

The possibility of nuclear weapons was acknowledged early in the war. At the University of Birmingham, Rudolf Peierls and Otto Robert Frisch co-wrote a memorandum explaining that a small mass of pure uranium-235 could be used to produce a chain reaction in a bomb with the power of thousands of tons of TNT. This led to the formation of the MAUD Committee, which called for an all-out effort to develop nuclear weapons. Wallace Akers, who oversaw the project, chose the deliberately misleading code name "Tube Alloys". His Tube Alloys Directorate was part of the Department of Scientific and Industrial Research.

The Tube Alloys programme in Britain and Canada was the first nuclear weapons project. Due to the high costs, and the fact that Britain was fighting a war within bombing range of its enemies, Tube Alloys was ultimately subsumed into the Manhattan Project by the Quebec Agreement with the United States, under which the two nations agreed to share nuclear weapons technology, and to refrain from using it against each other, or against other countries without mutual consent, although the United States did not provide complete details of the results of the Manhattan Project to the United Kingdom. The Soviet Union gained valuable information through its atomic spies, who had infiltrated both the British and American projects.

The United States terminated co-operation after the war ended, under the Atomic Energy Act of 1946. That prompted the United Kingdom to relaunch its own project, High Explosive Research. Production facilities were established and British scientists continued their work under the auspices of an independent British programme. Finally, in 1952, Britain performed a nuclear test under codename "Operation Hurricane". In 1958, in the wake of the Sputnik crisis, and the British demonstration of a two-stage thermonuclear bomb, the United Kingdom and the United States signed US–UK Mutual Defence Agreement, which resulted in a resumption of Britain's nuclear Special Relationship with the United States.