A September 1949 poll found that 72 per cent of Americans agreed that the United States should not "share our atomic energy secrets with England".[141] The reputation of the British was further tarnished by the 1950 revelation that Fuchs was a Soviet atomic spy.[141] British wartime participation in the Manhattan Project provided a substantial body of expertise that was crucial to the success of High Explosive Research, the United Kingdom's post-war nuclear weapons programme,[142] although it was not without important gaps, such as in the field of plutonium metallurgy.[143] The development of the independent British nuclear deterrent led to the McMahon Act being amended in 1958, and to a resumption of the nuclear Special Relationship between America and Britain under the 1958 US–UK Mutual Defence Agreement.[144][145]