British-US Communication Intelligence (UKUSA) Agreement

Overview
The British-US Communication Intelligence (UKUSA) Agreement was signed by representatives of the London Signals Intelligence Board and its American counterpart in March 1946. The UKUSA Agreement is without parallel in the Western intelligence world and formed the basis for co-operation between the two countries throughout the Cold War.

Source

 * The National Archives (UK), "Newly released GCHQ files: UKUSA Agreement" (June 2010) (full-text).