The tape, supposedly a recording of President Reagan and the then prime minister arguing over the progress of the Falklands War, was sent to a number of Dutch newspapers in the lead-up to the 1983 general election.
Submitted anonymously, a transcript claimed the president urged Mrs Thatcher “to control yourself”, to which the Iron Lady responded: “We have to use violence [on Argentina]. At this moment it is being used to punish them as quickly as possible.”
The audio and attached letter were quickly handed to the authorities and dismissed as a forgery.
Contemporary reports credited the work to punk-rock band Crass, but, according to Foreign and Commonwealth Office (FCO) documents released today, the government feared more subversive forces were at play.
In one letter, sent by an FCO adviser to Mrs Thatcher informing her of the incident, MI6 seemingly considered blaming the Soviet Union, Argentine secret services and left-wing radicals.
It stated: “The embassy in The Hague recently passed to London a tape recording of a purported telephone conversation between the prime minister and President Reagan during the Falklands crisis.”
The tape was exposed as a forgery through analysis by MI6, the CIA and Mrs Thatcher’s closest political advisors. According to a letter sent from Downing Street to the FCO, the recording was made with “voice-patch” technology, in which the pranksters cut together clips of the two leaders from public interviews.