Born into a non-observant Jewish family, he fled from Europe to the USA in 1940. In 1943 he became a member of the joint Anglo-Canadian atomic energy team at Montreal. In 1948 he obtained British citizenship and joined the British atomic bomb project at the Atomic Energy Research Establishment (AERE), in Harwell, Oxfordshire where he joined the Nuclear Physics Division as a principal scientific officer, led by Egon Bretscher.
In 1950 he was due to take up the position of chair of physics at Liverpool University but disappeared with his family after visiting Stockholm. His disappearance came just 10 months after another Harwell scientist, Klaus Fuchs, confessed to spying for the Soviet Union. The then British Minister of Supply, George Strauss, told MPs that the professor had had only limited access to "secret subjects" for some time, but he admitted it would have been possible for the professor to gather information, at Harwell or while he was in Canada, which could be "of value to the enemy".
Confirmation he had fled to Moscow came in March 1955 when he appeared at a news conference in Russia. He also wrote an article in Pravda in which he said he was involved in research into the use of atomic energy for peaceful purposes and had left the West because in his view it was intent on "new war using atomic and nuclear weapons as a means for achieving world domination".
Documents released by the Public Records Office under the 30-year rule revealed an administrative error had led to his being given security clearance to work at Harwell even though a search of his home in 1943 had uncovered many documents on communism. While in the USA he had not been asked to contribute to the Manhattan Project for the construction of the American atomic bomb, possibly because of his socialist beliefs.
He was awarded the Lenin prize for nuclear research in 1963.