It must have seemed a good idea at the time. A 15-minute light grilling on the morning BBC sofa with whichever stand-in presenter the corporation had dredged up to fill the void left by Andrew Marr, still recovering from a stroke. Nothing that an old hand like Boris Johnson need fear.

Tousle the hair a little, some self-deprecation and a bit of a plug for the BBC TV documentary on Monday to remind the Tory backbenchers that if the ball ever popped out of the scrum, he would be on hand to take it, almost accidentally, over the line. A spot of liberal differentiation from his school chum David Cameron on the benefits of migrants might provide with him an entry to the likely story of the day, the prime minister's imminent speech on migrants and access to social housing. But after the 15 minutes of chilling inquisition by the softly spoken Eddie Mair, Johnson's reputation had taken a severe pounding. Indeed, it was probably the worst interview the mayor has ever conducted.

It was inevitably described as a car crash, but in the case of Johnson, it was more of a bicycle crash: spokes all over the road, wheels mangled and a reputation badly dented.

After the opening exchanges – "Good morning, how are you?"; "Very, very good, thank you" – Johnson went downhill at an alarming pace until by the interview's close, admitting he had "sandpapered" quotes as a Times journalist, failing to deny he lied to the party leader at the time, Michael Howard, about an extramarital affair and conceding that he had humoured an old friend when he asked for a phone number in the knowledge that the friend intended to beat up the owner of it.

By the interview's close, "You're a nasty piece of work, aren't you?" was one of Mair's more generous reflections on Johnson's integrity.

Doubtless Johnson had been lulled into a false sense of security by the opening minutes in which he was able to hint, without providing incontrovertible proof, that he thought Cameron was misunderstanding the importance of migrants to the London economy.

He also gently put the boot into his predecessor as mayor for failing to plan the London Olympics' stadium properly. He came across as the charming, talented politician that he is.

But then Mair took the interview on an unexpected turn, and asked Johnson why he had agreed to be interviewed for the Michael Cockerell documentary. Johnson flannelled before, saying he had not seen the programme. Suddenly Mair's tone changed lethally: "But this happened in your life, so you know about this. The Times let you go after you made up a quote. Why did you make up a quote?"

It is impossible to describe the menacing politeness of tone in which Mair specialises, or his ability to pause mid-sentence to maximise the impact. Johnson asked plaintively: "Are you sure your viewers wouldn't want to hear more about housing in London?" It was, he added, a long and lamentable story, to which Mair replied: "OK. But you made a quote up."

Johnson was cornered. "Well, what happened was that … I ascribed events that were supposed to have taken place before the death of Piers Gaveston to events that actually took place after the death of Piers Gaveston," he said.

"Yes. You made something up," Mair replied. Johnson said: "Well, I mean, I mildly sandpapered something somebody said, and yes it's very embarrassing and I'm very sorry about it."

With this admission trousered, Mair continued: "Let me ask you about a barefaced lie. When you were in Michael Howard's team, you denied to him you were having an affair. It turned out you were and he sacked you for that. Why did you lie to your party leader?"

Johnson squirmed. "Well, I mean again, I'm … with great respect … on that, I never had any conversation with Michael Howard about that matter and, you know, I don't propose …"

Mair interrupted: "You did lie to him."

Johnson: "Well, you know, I don't propose to go into all that again."

Mair: "I don't blame you."

Johnson: "No, well why should I? I've been through, you know, that question a lot with the, well, watch the documentary. Why don't we talk about something else?"

Unfortunately for Johnson, Mair was willing to change the subject.

Referring to the documentary, Mair explained: "The programme includes your reaction as you listen to a phonecall in which your friend Darius Guppy asks you to supply the address of a journalist … so that he can have him physically assaulted. The words 'beaten up' and 'broken ribs' are said to you …"

Johnson replied after snorting about an old story being dragged up. "Yes, it was certainly true that he was in a bit of a state and I did humour him in a long phone conversation, from which absolutely nothing eventuated and … you know, there you go. But I think if any of us had our phone conversations bugged, they might, you know, people say all sorts of fantastical things whilst they're talking to their friends."

Mair proceeded to inform, in passing, a dazed Johnson that even convicted fraudster Conrad Black does not quite trust him, before asking him to show some honesty by openly admitting that his ambition is to be prime minister rather than trading in obfuscatory metaphors such as rugby balls emerging from a ruck or saying it is not going to happen.

Mair: "You're not going to land on the moon either. But do you want to be prime minister. Say it."

Johnson obfuscated, presumably hoping for something to eventuate, before saying he wanted to do all he could to help Cameron be re-elected – "and in those circumstances it is completelynonsensical for me to indulge, you know, this increasingly hysterical …"

Mair: "You could end it all just by saying what you know to be true. What should viewers make of your inability to give a straight answer to a straight question?"

By now most viewers are hiding behind their sofa, or telling their gawking children to look away, or ringing the BBC begging them to show the test card.

With the clock running down, Johnson desperately tries to mount a recovery, saying he disputes Mair's interpretations. Then he resorts to the old standby: "What viewers want to know is …"

He said: "They don't care about phone conversations with my friends 20 years ago, they don't care about some ludicrous, so-called made-up quote, and what's the third accusation? I can't remember …"

"Lying to Michael Howard," Mair reminds him, before Johnson finally collapses in a heap, his lights, pannier bag and reputation strewn across the bicycle lane.