MEANING BUSINESS.. Mrs Thatcher and Ronald Reagan in the Oval Office, November 1988

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The man, who had an English accent, said he was on his way to Camp David, the US President's retreat, but was lost. Running a licence check the ranger found the motorcycle had been stolen several days earlier and arrested the suspect. Frisking him, he found a revolver loaded with a single bullet, so foiling a bizarre plot to kill a VIP visitor: Britain's prime minister Margaret Thatcher. This 1986 incident was one of several threats to Thatcher's life that have come to light following the FBI's declassification of files relating to security during her many US trips.

Throughout her time at No 10, the IRA presented the greatest threat. In March 1979, weeks before she became PM, her political ally Airey Neave, who had masterminded her campaign to win the Tory leadership in 1975, was murdered by IRA splinter group the Irish National Liberation Army when his booby-trapped car exploded in the Commons car park. It wasn't long before Mrs Thatcher herself became a target for the Irish. Ahead of a 1981 US visit, an FBI informant told his handlers he had heard two men plotting to kill her. "On February 11, 1981, a source of proven reliability advised that he overheard two unknown subjects in the Boar's Head restaurant, Fall's Church, Virginia, discussing what may be a plot to assassinate Prime Minister Thatcher," said the report.

The informant heard one man remark: "This will even the score for 'H'. The Iron Maiden is no better than any other bloody PM". His companion said "the hit will be at the Asti. Lord 'C', too, if he's in the area." The FBI realised 'H' referred to H Block in Northern Ireland's Maze prison, where many IRA prisoners were held, while 'Asti' was New York's Waldorf-Astoria Hotel, where Mrs Thatcher and Foreign Secretary Lord Carrington were to stay. The information was considered highly credible and an investigation was launched.

OUTRAGE..Airey Neave and the remains of his car which was bombed by the IRA

Thatcher with her husband Dennis after attending the memorial service of Airey Neave

The FBI feared the assassin would strike at an anti-British demo by a pro-Irish Republican group outside the hotel on February 28. Known IRA sympathisers were investigated and the Boar's Head staked out. At least one suspect with IRA connections was interviewed but eliminated as a suspect. Despite the trail having gone cold Mrs Thatcher's US visit, on which she first met new US President Ronald Reagan, marking the beginning of their remarkable political partnership, passed without incident. However, Mrs Thatcher's hardline stance against the IRA ensured she remained their top target. In 1984 they very nearly killed her when they bombed the Tory Party conference at Brighton's Grand Hotel. However, there were also mentally unstable "lone wolves" for her security team to worry about, such as the English-born petty thief held by the park ranger in Maryland. He had convictions for theft and assault and, after trying to hang himself in a cell, was committed to a mental hospital. Later, he said he had intended to murder Mrs Thatcher. His grudge stemmed from a belief that, due to his record, he would be denied the right to return to the UK.

The US Secret Service said he "stated that since Mrs Thatcher was in charge of England and since he could not go to England, [he] decided to go to Camp David to shoot Mrs Thatcher." On December 11, 1989, another threat surfaced when the FBI's Washington DC office received a warning of a plot against President George Bush and Mrs Thatcher by a group calling itself God's Soldiers. The caller said "if President Bush and Prime Minister Thatcher are assassinated, their assassination would be worse than Jesus Christ being nailed to the cross." The FBI believed the call was made from the UK, but no more was ever heard. Even after Mrs Thatcher left office in November 1990, threats persisted, the most serious still from the IRA.