Interestingly for him, the harshest opposition to his proposals came from left-wing physicists, including Blackett from Imperial College and Powell, with whom he worked closely during the war. Their point was that the US would have a monopoly over nuclear research and would dictate matters until the Soviets developed nuclear weapons too.

Martin Underwood, Joseph Rotblat: a man of conscience in the nuclear age, Brighton [England] ; Portland, Or, 2009, p. 43