The Quebec Agreement was a secret agreement between the United Kingdom and the United States outlining the terms for the coordinated development of the science and engineering related to nuclear energy and specifically nuclear weapons. It was signed by Winston Churchill and Franklin D. Roosevelt on 19 August 1943, during World War II, at the First Quebec Conference in Quebec City, Quebec, Canada.

The Quebec Agreement stipulated that the US and UK would pool their resources to develop nuclear weapons, and that neither country would use them against the other, or against other countries without mutual consent, or pass information about them to other countries. It also gave the United States a veto over post-war British commercial or industrial uses of nuclear energy. The agreement merged the British Tube Alloys project with the American Manhattan Project, and created the Combined Policy Committee to control the joint project. Although Canada was not a signatory, the Agreement provided for a Canadian representative on the Combined Policy Committee in view of Canada's contribution to the effort.

British scientists performed important work as part of the British contribution to the Manhattan Project, and in July 1945 British permission required by the agreement was given for the use of nuclear weapons against Japan. The September 1944 Hyde Park Aide-Mémoire extended Anglo-American co-operation into the post-war period, but after the war ended, American enthusiasm for the alliance with Britain waned. The McMahon Act (1946) ended technical co-operation through its control of "restricted data". On 7 January 1948, the Quebec Agreement was superseded by a modus vivendi, an agreement which allowed for limited sharing of technical information between the United States, Britain and Canada.

The neutron was discovered by James Chadwick at the Cavendish Laboratory at the University of Cambridge in February 1932. In April 1932, his Cavendish colleagues John Cockcroft and Ernest Walton split lithium atoms with accelerated protons. Then, in December 1938, Otto Hahn and Fritz Strassmann at Hahn's laboratory in Berlin-Dahlem bombarded uranium with slowed neutrons, and discovered that barium had been produced. Hahn wrote to his colleague Lise Meitner, who, with her nephew Otto Frisch, explained that the uranium nucleus had been split. 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, in his 1913 novel The World Set Free. Sir Henry Tizard's Committee on the Scientific Survey of Air Defence was originally formed to study the needs of anti-aircraft warfare, but branched out to study air warfare generally. In May 1939, a few months before the outbreak of the Second World War in Europe in September 1939, it was directed to conduct research into the feasibility of atomic bombs. Tizard tasked George Paget Thomson, the professor of physics at Imperial College London, and Mark Oliphant, an Australian physicist at the University of Birmingham, with carrying out a series of experiments on uranium. By February 1940, Thomson's team had failed to create a chain reaction in natural uranium, and he had decided that it was not worth pursuing.

Oliphant's team reached a strikingly different conclusion. He had delegated the task to two German refugee scientists, Rudolf Peierls and Frisch, who could not work on the university's secret projects like radar because they were enemy aliens, and therefore lacked the necessary security clearance. 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 1 to 10 kilograms (2.2 to 22.0 lb) would suffice, and would explode with the power of thousands of tons of dynamite.

Oliphant took the Frisch–Peierls memorandum to Tizard. As a result, the MAUD Committee was established to investigate further. It directed an intensive research effort. Four universities provided the locations where the experiments were taking place. The University of Birmingham undertook theoretical work, such as determining what size of critical mass was needed for an explosion. This group was run by Peierls, with the help of fellow German refugee scientist Klaus Fuchs. The laboratories at the University of Liverpool and the University of Oxford experimented with different types of isotope separation. Chadwick's group at Liverpool dealt with thermal diffusion, a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperature gradient. Francis Simon's group at Oxford investigated the gaseous diffusion, which works on the principle that at differing pressures uranium 235 would diffuse through a barrier faster than uranium 238. This was determined to be the most promising method. Egon Bretscher and Norman Feather's group at Cambridge investigated whether another element, now called plutonium, could be used as a fissile material. Because of the presence of a team of refugee French scientists led by Hans von Halban, Oxford also had the world's main supply of heavy water, which helped them theorise how uranium could be used for power.

In July 1941, the MAUD Committee produced two comprehensive reports that concluded 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 MAUD Committee unanimously recommended pursuing its development as a matter of urgency, although it recognised that the resources required might be beyond those available to Britain. But even before its report was completed, the Prime Minister, Winston Churchill, had been briefed on its findings by his scientific advisor, Frederick Lindemann, and had decided on a course of action. A new directorate known by the deliberately misleading name of Tube Alloys was created to co-ordinate 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 its director.