THE startling truth of Britain's most closely guarded secret of the Cold War has at last been learned. Four RAF pilots were decorated for their part in the CIA's U2 spyplane missions over the Soviet Union.

Until now, the idea that RAF personnel took part in the flights, which brought East-West tension to one of its most dangerous peaks when the American pilot Gary Powers was shot down and captured in 1960, would have seemed fantasy.

Nearly forty years on, the Government is still refusing to admit the RAF's involvement with the revolutionary long-winged spy plane which, before the age of satellites, was used to photograph Soviet missile sites by daringly flying right across the country.

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Ministry of Defence files on the controversial CIA operations are being withheld from the Public Records Office in Kew and, remarkably, the British refusal is preventing the publication of the CIA's own history of the controversial project.

Asked about the RAF's involvement with the U2 saga late last week, the MoD would only say: "The MoD is not in a position to make any comment on the operation of U2 aircraft by the US Government."

It is indeed generally believed that only Americans flew the CIA U2s, but in fact four RAF pilots - Squadron Leader Robert Robinson and Flight- Lieutenants Michael Bradley, David Dowling and John MacArthur - were attached to the CIA to fly the aircraft, and were awarded the Air Force Cross for doing so.

But their citations in the London Gazette (27 December 1960 and 29 December 1961) make no mention of their provocative but courageous spying missions.

The RAF men made several flights, each personally approved by the then Prime Minister Harold Macmillan, across Soviet Russia to photograph rocket sites. The exact number is still a closely guarded secret.

The leader of the RAF U2 detachment, Wing Commander (as he became) Robert Robinson, spoke of his role before he died last year. He told me: "In 1958 this was the most secret operation in the world and the British involvement most secret of all." Robinson was paid through a secret MI6 bank account.

The U2 project had been approved in 1954 by President Eisenhower to overcome severe difficulties obtaining intelligence from behind the Iron Curtain. In effect a jet-engined glider loaded with cameras, the U2 could fly at an unprecedented 70,000 feet. The first missions were flown from Germany in early July 1956. Then security concerns led to further missions being flown from Turkey, Pakistan or Japan. But by 1958 Eisenhower was worried that the U2 would sour US-USSR relations and it was becoming difficult for the CIA to get his permission for overflights of the USSR.

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Richard Bissell, CIA officer in charge, suggested bringing the British into the programme to increase the number of overflights. "My theory was to set up a system whereby there would be another chief of state who could give consent, namely the British Prime Minister. So I approached the RAF, and needless to say, they were eager to be in on the act," he said later.

Harold Macmillan agreed the plan, and four RAF officers were selected and sent to Laughlin Air Force Base in Del Rio, Texas, for training in May 1958. They were Squadron Leader Christopher Walker, along with Bradley, Dowling and MacArthur.

On 8 July 1958, Sq Ldr Walker was killed when his U2 crashed near Wayside, Texas. In his place Sq Ldr Robert Robinson was brought into the project. After extensive training Robinson was sent to Incirlik Air Force Base in Turkey in January 1959 where the CIA's U2 "Detachment B" was based. This consisted of four aircraft, seven US "civilian" pilots and about 200 support personnel from the CIA.

The official cover story for the British was that they were temporary employees of the Meteorological Office in London. The U2's ability to fly so high protected it from Soviet aircraft. The RAF pilots regularly flew over the Middle East and other trouble spots. Occasionally they flew over the Soviet Union despite attempts to shoot them down.

Robinson told me: "You were always looking behind, and you would see many, many aircraft all lined up below you but with the inability to reach you."

How many Soviet overflights did the British undertake? This is one area where Robinson remained coy. In the autumn of 1995 the CIA finally declassified the number of U2 overflights of the USSR - 24. The number of overflights flown by the British remains a secret, but it is between two and four - all personally approved by the Prime Minister. In 1959 Sq Ldr Robinson flew over two Soviet rocket testing sites. Fl Lt MacArthur flew another overflight in early 1960.

The U2 project made headline news across the world when Gary Powers' plane was shot down over Sverdlovsk in the USSR with a SAM-2 missile on May Day 1960. As Eisenhower suspected, a storm was unleashed and Nikita Kruschev, the Soviet leader, cancelled his summit with the Americans.

Robinson warned the US detachment commander that May Day was not a good day to fly over the USSR. "There'd be a maximum alert and it was a very dangerous thing to do, highly provocative, as it turned out. It was getting very close to Moscow and I would think they were very nervous. They knew that something was happening and clearly the order was given it must be destroyed."

After three days of uncertainty the news that Powers had been captured meant the British unit quickly packing up and leaving Turkey. "As soon as it was known that he'd been captured the pilots left immediately. This was really to save the embarrassment with the Turkish government because they didn't know we were there anyway and it was best we left," said Robinson.

The British detachment was instructed to "vanish" until things died down. Robinson went to Spain for some months.

Later, a limited version of the CIA's U2 programme was resuscitated, although there were to be no permanent overseas units and there were no overflights of the Soviet Union.

British involvement continued. In 1961 another two RAF pilots, Sq Ldr Ivan "Chunky" Webster and Fl Lt Charles Taylor, joined the CIA programme. They were replaced in 1964 by Sq Ldr Basil Dodd and Fl Lt Martin Bee. However, Webster lobbied to stay in the U2 programme and resigned from the RAF to be hired by the Lockheed Corporation to continue flying the U2. Other RAF officers flew U2s before British involvement in the project ended in the late 1960s.

Wing Commander Robinson has been the only RAF pilot to speak of his experience. It is hard to understand why, given his testimony and the CIA's openness about the U2, as well as the ending of the Cold War - but the MOD will not release its files on the subject.

Paul Lashmar's 'Spy Flights of the Cold War', Sutton Publishing Ltd, pounds 18.99.