'You are a nasty piece of work, aren't you?': Boris Johnson's father Stanley attacks Eddie Mair over interview on Andrew Marr Show

London Mayor left floundering by a series of damaging questions from BBC presenter

Boris Johnson seen immediately after his interview with Eddie Mair (PA)

Boris Johnson’s past troubles finally returned to haunt him yesterday when he gave what senior Conservatives called a “car crash” television interview which they said had dented his hopes of becoming Conservative Party leader.

The colourful, humorous Mayor of London is often treated with kid gloves by interviewers but Eddie Mair, who hosted the BBC’s Andrew Marr Show, was wearing boxing gloves. He subjected Mr Johnson to the awkward questions and scrutiny he could expect if he were running to be Prime Minister rather than running the capital.

“You’re a nasty piece of work, aren’t you?” asked Mr Mair. Mr Johnson looked surprised and distinctly uncomfortable as the presenter asked him about his being fired by The Times newspaper for making up a quotation; being sacked from the Tory frontbench for telling “a bare-faced lie” to the party leader Michael Howard about his affair with the journalist Petronella Wyatt and the claim that he agreed to provide a reporter’s address to his friend Darius Guppy, a convicted fraudster, so the journalist could be beaten up.

An exasperated Mr Johnson said he would “dispute” all three allegations. Only up to a point, it seemed. He admitted he had “mildly sandpapered something someone said” for The Times, saying it was “very embarrassing” and he was “very sorry”. He insisted he never spoke directly to Mr Howard about the affair, although at the time in 2004 he publicly denied it as “an inverted pyramid of piffle.” He told Mr Mair: “I could explain that I think all three interpretations you’re putting on those things aren’t wholly fair…. the case of my old friend Darius [Guppy] yes, it was certainly true that he was in a bit of state and I did humour him in a long phone conversation, from which absolutely nothing eventuated. But I think if any of us had our phone conversations bugged, they might, you know, people say all sorts of fantastical things whist they’re talking to their friends.”

The softly-spoken, dry but deadly BBC presenter, standing in for Mr Marr while he recovers from a stroke, told Mr Johnson: “I want to talk about you.” Revealingly, the Mayor replied: “Well that’s exactly what I’m trying to avoid.” He insisted it was “nonsensical” to talk about whether he wanted to be Prime Minister and insisted he wanted David Cameron to win the next election, even though some Tory MPs dream of installing Mr Johnson as party leader before or after the 2015 poll.

The Mayor complained that he had in effect been blackmailed by Michael Cockerell, the veteran documentary-maker, into co-operating with a programme about him to be screened by the BBC tonight

Asked about his ambitions, Mr Johnson told Mr Cockerell: “I think it’s a very tough job being Prime Minister. Obviously if the ball came loose from the back of the scrum – which it won’t— it would be a great, great thing to have a crack at it. But it’s not going to happen.”

Some of those interviewed by Mr Cockerell were less than flattering about his subject. “Boris isn’t pretending to be chaotic; he really is utterly chaotic,” said Sir Max Hastings, his former editor at the Daily Telegraph. Conrad Black, who appointed Mr Johnson editor of The Spectator magazine, describes him as “a sly fox disguised as a teddy bear.”

Yesterday Mr Mair had the advantage because he had already watched a sneak preview of “Boris Johnson: The Irresistible Rise”, to be shown at 9pm tonight on BBC2. “I’m not going to watch it,” Mr Johnson told him. “No. I’m certainly not, not after what you’ve told me.”

And this morning Johnson's father, Stanley Johnson, defended his son, telling Nick Ferrari on LBC radio: "I thought Eddie Mair's interview was about the most disgusting piece of journalism I've listened to for a very long time."

"The BBC sank about as low as it could. If grilling people about their private lives, accusing them of guilt by association and openly abusing them is a legitimate interview, then frankly, I don't know where we are coming."

But later this morning Johnson himself said: "Eddie Mair did a splendid job. There is no doubt that is what the BBC is for - holding us to account.

"He was perfectly within his rights to have a bash at me - in fact it would have been shocking if he hadn't. If a BBC presenter can't attack a nasty Tory politician what's the world coming to?"