The United Kingdom became the third country (after the United States and the Soviet Union) to develop and, in 1952, test nuclear weapons. The UK is one of nine nuclear-armed states, and one of five recognized by the Treaty on the Non-Proliferation of Nuclear Weapons. , the UK possesses a stockpile of approximately 225 warheads, with 120 deployed on its only delivery system, the Trident programme's submarine-launched ballistic missiles. Additionally, United States B61 nuclear bombs have potentially been stored at RAF Lakenheath since 2025. In 2025, the UK announced plans to procure 12 F-35A aircraft capable of delivering B61s.

Trident consists of four s, based at HMNB Clyde in Scotland. Each is armed with up to sixteen Trident II missiles, each carrying warheads in up to eight multiple independently targetable re-entry vehicles. Since 1969, the Royal Navy has operated the continuous at-sea deterrent: at least one nuclear-armed ballistic missile submarine always on patrol. Under the Polaris Sales Agreement, the US supplied the UK with Polaris missiles and nuclear submarine technology, in exchange for the general commitment of these missiles to NATO. In 1982, an amendment allowed the purchase of Trident II missiles, and since 1998, Trident has been the only British nuclear weapons system. The UK is the only nuclear-armed country to use exclusively sea-launched nuclear weapons.

UK nuclear command and control launch authority rests with the Prime Minister, or if unreachable a designated deputy, while it is possible the Chief of the Defence Staff holds a veto. In the event of a decapitation strike, handwritten letters of last resort from the Prime Minister are used to instruct submarine crews. UK nuclear weapons have no permissive action links or corresponding launch codes, thus submarine crews may independently launch a nuclear attack.

The UK initiated the world's first nuclear weapons programme, Tube Alloys, in 1941 during the Second World War. At the 1943 Quebec Conference, it was merged with the American Manhattan Project, but collaboration ended in 1946. The UK began an independent programme, High Explosive Research, testing its first nuclear weapon in 1952. In total the UK conducted 45 nuclear tests, 12 in Australia, 9 in the Pacific, and 24 at the Nevada Test Site, with its last in 1991. The UK and France are the only two nuclear-armed countries that have ratified the Comprehensive Nuclear-Test-Ban Treaty.

The British hydrogen bomb programme's success with its Operation Grapple Pacific nuclear testing led to the 1958 US–UK Mutual Defence Agreement. This nuclear Special Relationship has involved the exchange of classified scientific data, warhead designs, and fissile materials. UK warheads are designed and manufactured by the Atomic Weapons Establishment. During the Cold War, the Royal Air Force operated the V bomber fleet for strategic weapons, followed by aircraft in tactical nuclear roles using WE.177 bombs. The RAF also planned to operate the cancelled Blue Streak intermediate-range ballistic missile (IRBM). The RAF also briefly operated Thor IRBMs under US custody, while both the RAF and the British Army of the Rhine operated US-custody tactical bombs, missiles, depth charges and artillery. US Air Force nuclear weapons were stationed in the UK between 1954 and 2008, and from 2025.

History

Tube Alloys

The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932, and in April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. In December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slow neutrons, and discovered that barium had been produced. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, determined that the uranium nucleus had been split, a conclusion they published in Nature in 1939. By analogy with the division of biological cells, they named the process "fission".

The discovery of fission raised the possibility that an extremely powerful atomic bomb could be created. The term was already familiar to the British public through the writings of H. G. Wells, with a continuously exploding bomb in his 1913 novel The World Set Free. George Paget Thomson, at Imperial College London, and Mark Oliphant, an Australian physicist at the University of Birmingham, were tasked with carrying out a series of experiments on uranium. Oliphant delegated the task to two German refugee scientists, Rudolf Peierls and Frisch, who ironically could not work on the university's secret projects like radar because they were enemy aliens and therefore lacked the necessary security clearance. In March 1940 they calculated the critical mass of a metallic sphere of pure uranium-235, and found that instead of tons, as everyone had assumed, as little as would suffice, which would explode with the power of thousands of tons of dynamite.

Oliphant took the resulting Frisch–Peierls memorandum to Sir Henry Tizard, the chairman of the Tizard Committee, and the MAUD Committee was established to investigate further. It held its first meeting on 10 April 1940, in the ground-floor main committee room of the Royal Society in Burlington House in London. It directed an intensive research effort, and in July 1941, produced two comprehensive reports that reached the conclusion that an atomic bomb was not only technically feasible, but could be produced before the war ended, perhaps in as little as two years. The Committee unanimously recommended pursuing the development of an atomic bomb as a matter of urgency, although it recognised that the resources required might be beyond those available to Britain. A new directorate known as Tube Alloys was created to coordinate this effort. Sir John Anderson, the Lord President of the Council, became the minister responsible, and Wallace Akers from Imperial Chemical Industries (ICI) was appointed the director of Tube Alloys.

Manhattan Project

In July 1940, Britain had offered to give the United States access to its scientific research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American S-1 Project (later renamed the Manhattan Project) was smaller than the British, and not as far advanced. The British and American projects exchanged information, but did not initially combine their efforts. British officials did not reply to an August 1941 American offer to create a combined project. In November 1941, Frederick L. Hovde, the head of the London liaison office of the American Office of Scientific Research and Development (OSRD), raised the issue of cooperation and exchange of information with Anderson and Lord Cherwell, who demurred, ostensibly over concerns about American security. Ironically, it was the British project that had already been penetrated by atomic spies for the Soviet Union.

The United Kingdom did not have the manpower or resources of the United States, and despite its early and promising start, Tube Alloys fell behind its American counterpart and was dwarfed by it. On 30 July 1942, Anderson advised the Prime Minister of the United Kingdom, Winston Churchill, saying: "We must face the fact that ... [our] pioneering work ... is a dwindling asset and that, unless we capitalise it quickly, we shall be outstripped. We now have a real contribution to make to a 'merger.' Soon we shall have little or none."

The British considered producing an atomic bomb without American help, but it would require overwhelming priority, would disrupt other wartime projects, and was unlikely to be ready in time to affect the outcome of the war in Europe. The unanimous response was that before embarking on this, another effort should be made to secure American cooperation. At the Quebec Conference in August 1943, Churchill and the President of the United States, Franklin Roosevelt, signed the Quebec Agreement, which merged the two national projects. The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to coordinate their efforts, and specified that the weapons could only be used if both the US and UK governments agreed. The 19 September 1944 Hyde Park Agreement extended both commercial and military cooperation into the post-war period.

A British mission led by Akers assisted in the development of gaseous diffusion technology at the SAM Laboratories in New York. Another, led by Oliphant, who acted as deputy director at the Berkeley Radiation Laboratory, assisted with the electromagnetic separation process. Cockcroft became the director of the Anglo-Canadian Montreal Laboratory. The British mission to the Los Alamos Laboratory was headed by Chadwick, and later Peierls. It included distinguished scientists such as Geoffrey Taylor, James Tuck, Niels Bohr, William Penney, Frisch, Ernest Titterton and Klaus Fuchs, who was later revealed to be a Soviet spy. As overall head of the British Mission, Chadwick forged a close and successful partnership with Brigadier General Leslie R. Groves, the director of the Manhattan Project. He ensured that British participation was complete and wholehearted.

Penney worked on means to assess the effects of a nuclear explosion, and wrote a paper on what height the bombs should be detonated at for maximum effect in attacks on Germany and Japan. He served as a member of the target committee established by Groves to select Japanese cities for atomic bombing, and on Tinian with Project Alberta as a special consultant. Because the Quebec Agreement specified that nuclear weapons would not be used against another country without mutual consent, British authorisation was required for their use. On 4 July 1945, Field Marshal Henry Maitland Wilson agreed that the use of nuclear weapons against Japan would be recorded as a decision of the Combined Policy Committee. Along with Group Captain Leonard Cheshire, sent by Wilson as a British representative, Penney watched the bombing of Nagasaki from the observation plane Big Stink. He also formed part of the Manhattan Project's post-war scientific mission to Hiroshima and Nagasaki that assessed the extent of the damage caused by the bombs.

End of American cooperation

With the end of the war, the Special Relationship between Britain and the United States "became very much less special". The British government had trusted that America would share nuclear technology, which it considered a joint discovery. On 8 August 1945, the Prime Minister, Clement Attlee, sent a message to President Harry Truman in which he referred to themselves as "heads of the Governments which have control of this great force". But Roosevelt had died on 12 April 1945, and the Hyde Park Agreement was not binding on subsequent administrations. In fact, it was physically lost. When Wilson raised the matter in a Combined Policy Committee meeting in June, the American copy could not be found.

On 9 November 1945, Attlee and the Prime Minister of Canada, Mackenzie King, went to Washington, D.C., to confer with Truman about future cooperation in nuclear weapons and nuclear power. A Memorandum of Intention they signed replaced the Quebec Agreement. It made Canada a full partner, continued the Combined Policy Committee and Combined Development Trust, and reduced the obligation to obtain consent for the use of nuclear weapons to merely requiring consultation. The three leaders agreed that there would be full and effective cooperation on atomic energy, but British hopes were soon disappointed; Groves restricted cooperation to basic scientific research.