Joseph Rotblat was born in Warsaw, 4 November 1908, the son of Zygmunt Rotblat and Sonia (née Krajtman). He worked from the age of twelve, as an apprentice to an electrician, but studied at night and won a place to read Physics at the Free University of Poland. He married Tola Gryn in the 1930s (died 1942).

He was a research fellow at the Radiological Laboratory of the Scientific Society of Warsaw, from 1932, where he gained his PhD, 1938, and assistant director of the Atomic Physics Institute at the Free University of Poland, 1937-9. He joined the staff of Liverpool University, 1939, working with James Chadwick's research group on the early development of an atomic weapon, and then on the Manhattan Project at Los Alamos, March 1944. He decided to leave the project for ethical reasons, later in 1944, and returned to Liverpool University, where he resumed his work as a physicist and from where he received a PhD, in 1950. After the war, he discovered that his wife, Tola, who had been unable to escape from Poland, had died in Majdanek Concentration Camp, probably in 1942. He settled permanently in Britain and was joined by the surviving members of his family.

He began to campaign against atomic weapons: co-founding the Atomic Scientists' Association, 1946, and instigating the Atom Train, an exhibition that toured Britain, 1947-8, and later Europe and the Middle East. In 1949, he was appointed Professor of Medical Physics at St Bartholomew's Hospital, London, where he worked until his retirement in 1976. His research focused on the application of nuclear physics to medicine and the effects of radiation on living organisms. He was editor-in-chief of "Physics in Medicine and Biology", 1960-72.

He continued to campaign against weapons of mass destruction and war and, in 1955, he was a signatory of the Russell-Einstein Manifesto, which led to the first of the Pugwash Conferences on Science and World Affairs, 1957, named after the small town in Nova Scotia where the meeting took place. He became the moving spirit of the Pugwash organisation, co-ordinating and attending many of its meetings, which brought together scientists from around the world. He served in the following offices within the organisation: Secretary-General, 1957-73; Chairman of British Pugwash, 1978-88; and President of International Pugwash, 1988-97.

He and the Pugwash organisation were jointly awarded the Nobel Prize for Peace, 1995. He was also awarded a CBE, 1965, and a knighthood, 1998.

He died on 31 August 2005.

His publications include: "Progress in Nuclear Physics" (1950); with James Chadwick, "Radio-activity and Radioactive Substances" (1953); "Atomic Energy, a Survey" (1954); "Atoms and the Universe" (1956); "Science and World Affairs" (1962); "Aspects of Medical Physics" (1966); "Pugwash, the First Ten Years" (1967); "Scientists in the Quest for Peace" (1972); "Nuclear Reactors: to Breed or Not to Breed" (1977); "Nuclear Energy and Nuclear Weapon Proliferation" (1979); "Nuclear Radiation in Warfare" (1981); "Scientists, the Arms Race and Disarmament" (1982); "The Arms Race at a Time of Decision" (1984); "Nuclear Strategy and World Security" (1985); "World Peace and the Developing Countries" (1986); "Strategic Defence and the Future of the Arms Race" (1987); "Coexistence, Co-operation and Common Security" (1988); "Verification of Arms Reductions" (1989); "Nuclear Proliferation: Technical and Economic Aspects" (1990); "Global Problems and Common Security" (1990); "Towards a Secure World in the 21st Century" (1991); "Striving for Peace, Security and Development in the World" (1992); "A Nuclear Weapon-Free World: Desirable? Feasible?" (1993); "A World at the Crossroads: New Conflicts, New Solutions" (1994); "Towards a War-Free World" (1995); "World Citizenship: Allegiance to Humanity" (1996); "Nuclear Weapons: the Road to Zero" (1998); "Eliminating the Causes of War" (2001); and with Robert Hinde, "War No More" (2003).