The future shadow higher education minister, who was then a trainee on The Times, decided to spice up the find with references to gay sex among the royals and then “invented” a comment from a distinguished Oxford don.

The quote from Sir Colin Lucas, now chairman of the British Library, claimed that the king “had enjoyed a reign of dissolution with his catamite, Piers Gaveston” in the palace, which was built in 1325.

Unfortunately for Johnson, who is now tasked with presenting Tory policy on universities, Gaveston was beheaded in 1312.

A new biography of the man once tipped as the next Conservative prime minister, serialised in today’s News Review, says that “at high tables the length and breadth of the kingdom, a roar of laughter went up at Colin Lucas’s expense . . . every sub-lecturer from Sheffield wrote in to point out his error”.

To make matters worse Lucas was Johnson’s godfather and an expert on the French revolution, not medieval England.

Andrew Gimson, the author of the book, who says that Johnson offered him £100,000 to abandon the biography, writes: “Boris had scored a hit, but it was not the hit he had intended. He had managed to sex the story up — a catamite, after all, is a boy kept for homosexual purposes — but had also committed a grotesque historical error.”

He claims that Johnson was unable to contact Lucas to check he was happy for his name to be used in this way. Lucas, who was trying unsuccessfully at the time to become master of Balliol College, complained to The Times. Undeterred, Johnson wrote a second story about the discovery of the building.

This time Lucas was quoted as saying: “Edward II is reputed to have led a life of wine and song with his catamite Piers Gaveston. But if 1325 is correct, that could hardly have taken place in this building since Gaveston was executed in 1312.”

The contorted quote was little recompense for Lucas, who was later to become vice-chancellor of Oxford University. Johnson was summoned to appear before Charles Wilson, then editor of The Times, and later dismissed for making up quotes.

This weekend Johnson insisted that he did speak to Lucas before writing the original story but had wrongly attributed his own mistake about Gaveston to him. “I talked to various archeologists and historians,” he said. “But I was asked to provide detail about Edward II. In desperation, I rang up Colin. He brilliantly extemporised some stuff about silken-haired youths and Piers Gaveston which I put in.

“The problem was that the castle had not been constructed while Gaveston was still alive. A lot of whingeing, snivelling, fact-grubbing historians wrote asking sarcastically was this the same Colin Lucas who was an expert on the French revolution.

“I had applied Colin’s description of life in Edward II’s court to the palace which he did not intend. Colin showed his ruthlessness in vindicating the accuracy of his remarks.”

Wilson said: “At the time Boris didn’t seem to take making up quotes as a very serious matter. I thought it was a heinous crime.”

Johnson was then hired by the Daily Telegraph and became the paper’s Brussels correspondent. There rival correspondents claimed he made up entire stories, including one that the EU planned to blow up the commission’s asbestos-clad Berlaymont headquarters with dynamite.

These made him Margaret Thatcher’s favourite journalist and he was able to launch a political career, becoming MP for Henley-on-Thames in 2001 as well as editor of The Spectator. He resigned from the magazine when David Cameron recalled him to the Tory front bench last December.

His trajectory to the top had been blown off course in 2004 with the revelation of his affair with Petronella Wyatt. Gimson writes that she had two abortions after becoming pregnant by him.

Johnson’s public image as a tousle-haired scruff with a studied vagueness will not be helped by the biography’s description of his first wedding, to Allegra Mostyn-Owen.

He arrived without the right clothes and got married wearing the trousers and cufflinks of John Biffen, the former Tory minister. He then lost his wedding ring within the hour and forgot he had stuffed his marriage certificate in the trousers he had returned to Biffen.

Last week, true to form, he was forced to apologise to the people of Papua New Guinea for describing them as prone to “cannibalism and chief-killing”.

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