The defection of Igor Gouzenko and the resulting espionage conviction of Alan Nunn May, a British physicist who had worked at the Montreal Laboratory, made it politically impossible for US officials to exchange information with the UK. Congress, unaware of the Hyde Park Aide-Mémoire because of the loss of the American copy, enacted the McMahon Act. Signed by Truman on 1 August 1946, and in effect from midnight on 1 January 1947, this law ended technical co-operation. Its control of "restricted data" prevented the United States' allies from receiving any information. The remaining scientists were denied access to papers that they had written just days before. The McMahon Act fuelled resentment from British scientists and officials alike, and led directly to the British decision in January 1947 to develop its own nuclear weapons. In the United States, there was a furore over the British veto over the use of nuclear weapons when the Joint Committee on Atomic Energy was informed of the Quebec Agreement (but not the November 1945 agreement) on 12 May 1947, resulting in intense pressure on Truman to drop the provision. On 7 January 1948, Bush, James Fisk, Cockcroft and Mackenzie concluded an agreement known as the "modus vivendi", that allowed for limited sharing of technical information between the United States, Britain and Canada, which officially repealed the Quebec Agreement. Like the Quebec Agreement it replaced, the "modus vivendi" was classified "Top Secret".