The UKUSA agreement, signed on 5 March 1946, brought British, US and Commonwealth codebreakers together in a unique alliance that operates to this day. Yet over the next half century, sharing was by no means 100% and was often accompanied by acrimony.
GCHQ and its American partner, the National Security Agency, swapped intelligence on the Soviet Union and China. They also worked together on eastern European states, known as Exotics. But elsewhere, typically in the Middle East, London and Washington clashed over foreign policy and did not share much signals intelligence.
Access to intelligence was sometimes a useful stick for beating allies. In August 1973, Richard Nixon and Henry Kissinger lost patience with Edward Heath's pro-European polices. To signal their displeasure, they told their people to cut Britain out.
The agency protested that intelligence exchange with GCHQ was governed by the UKUSA agreement and such a "cut-off" was awkward. Nevertheless, many streams of intelligence, especially imagery from satellites and aircraft, dried up and Britain's Joint Intelligence Committee panicked.
In October 1973, the Yom Kippur war offered Heath the opportunity to retaliate – which he relished. He placed onerous restrictions on US intelligence activities and over-flights from bases in Britain and Cyprus. The ensuing arguments were not resolved until Nixon was brought down by Watergate in 1974.
Canada also endured "cut-offs". After Saddam Hussein's invasion of Kuwait in 1990, Washington asked Ottawa to assist by sending naval ships to the Gulf. The Canadian fleet was out-dated and equipped for anti-submarine warfare. Fearing the threat from aircraft and Exocet missiles, the Canadians protested that their ships would be too vulnerable.
Washington signalled its intense displeasure by cutting off the intelligence flow and so the "screens went blank". Ottawa had a change of heart and three days later communications were restored. In honour of this memorable episode in allied relations, Ottawa's defence chiefs christened their Gulf naval deployment "Operation Friction".
The latest twist in this complex tale is the use of intelligence power to limit inquiries into abuses by the secret services. In 2005, the Americans shut off the flow of intelligence once more because Canada had set up an inquiry into the case of Maher Arar, a citizen who had been the victim of rendition to Jordan and Syria. The inquiry team had been allowed to look at classified American material – against Washington's wishes.
This February, it was the turn of Britain's then foreign secretary, David Miliband, to feel discomfort. When British judges wished to make public 25 lines of text derived from American intelligence reports, he asserted that this would contravene the ''control principle" that governs the intelligence-sharing relationship. Nevertheless, he claimed that there had been "no threat from the US to break off intelligence co-operation".
Richard Aldrich is professor of international studies at Warwick University, is the author of GCHQ, The uncensored story of Britain's most secret intelligence agency