ABSTRACT
International pressure for an end to atmospheric nuclear testing in the mid and late 1950s coincided with a key moment in the development of the British nuclear weapons programme. The Cabinet Defence Committee decided in 1954 that the UK should possess thermonuclear weapons and Aldermaston embarked on a crash programme to meet this requirement. Domestic opposition to nuclear testing was an important factor in shaping British policy in the mid and late 1950s. Moreover, it had become very clear to the UK at the end of 1956, following the conversations that Penney had in Washington on his way home from the Mosaic trials, that the key to access US nuclear secrets was that the British scientists should know how to make megaton weapons. Penney noted in June 1957 that one of the most important points that AWRE had understood from the Grapple tests was how to make a thermonuclear bomb.
