On 5 August 1963, Britain, the United States and the Soviet Union signed a treaty in Moscow banning nuclear explosions in the atmosphere, outer space and underwater.

The signatories were British Foreign Secretary Alec Douglas-Home, United States Secretary of State Dean Rusk and Soviet Foreign Minister Andrei Gromyko. The agreement became commonly known as the Partial Test Ban Treaty.

Nuclear powers had carried out hundreds of weapons tests since 1945. Many took place above ground or over the sea. These explosions released radioactive material into the atmosphere, where it could travel far beyond the test site.

Britain had tested nuclear weapons in Australia and the Pacific. The United States and Soviet Union had conducted much larger programmes. Scientific evidence and public concern about radioactive fallout increased during the 1950s and early 1960s.

The three governments had discussed a test ban for several years. They agreed that testing should be restricted but remained divided over how compliance could be verified.

Underground explosions created the main difficulty. The United States and Britain wanted inspections that could distinguish nuclear tests from earthquakes and other seismic events. The Soviet Union opposed an inspection system it regarded as a possible means of espionage.

The treaty avoided that disagreement by covering environments in which nuclear explosions were easier to detect. It prohibited tests in the atmosphere, including explosions over land and sea. It also prohibited them underwater and in outer space.

Underground tests remained permitted provided that radioactive debris did not spread beyond the borders of the country conducting the explosion. The agreement restricted testing rather than ending it.

The treaty covered nuclear explosions described as peaceful as well as weapons tests. Governments could not avoid its terms by assigning a civil purpose to an explosion.

It entered into force on 10 October 1963. Other countries were invited to join, and the agreement gained wide international support. France and China did not sign at the time and continued conducting atmospheric tests.

The treaty reduced the radioactive fallout produced by above-ground testing. Britain, the United States and the Soviet Union continued developing and testing nuclear weapons underground.