Boris Johnson, the good time politician with the amiable air of a man who never cracks under pressure, completely lost it today.

After months of stressful campaigning in the run up to May’s London mayoral election, he finally let his rival, Ken Livingstone get under his skin.

In a crowded lift, in front of three witnesses, the Mayor met the ex-Mayor in a nose to nose confrontation, and called him a “fucking liar” three times over. He was, reputedly, red with rage.

The explosion occurred after the four main candidates in the mayoral contest had taken part in a hustings hosted by the LBC radio station. It was witnessed by the Liberal Democrat candidate, Brian Paddick, the Green, Jenny Jones, and an LBC executive.

After the programme, the five of them got into a second floor lift to make a 30 second journey for a photocall on the roof. They emerged in front of cameras as if nothing had happened. News of the Boris tantrum leaked out soon afterwards.

London’s Mayor is one of the sharpest performers in the politics, who has got the better of hard nosed interviewers like Jeremy Paxman and kept cool while being taunted by the professional comics on Have I Got News For You. Today’s outburst prompted the question whether he feels the London mayoralty slipping away to his older opponent – because defeat in May would be a shattering setback for one of the most ambitious men in modern politics.

He was possibly provoked by the Twitter poll that LBC was running while the four candidates were on air. Half way through, it was showing Johnson and Livingstone in front on 25 per cent each, but at the end of the hour, Livingstone and Paddick were level on 24 per cent while Johnson was trailing on 16. The poll is unscientific and self-selecting, but it allowed Livingstone’s people to claim that Johnson had lost his rag because he had lost the debate.

Also, it should be said in Johnson’s defence that very few politicians who can match Livingstone’s special talent for driving others to fury, partly through the manner in which he makes outrageous accusations in that nasal London twang.

Johnson’s people say that what actually made him angry was being accused of something that was simply untrue. The single most damaging setback for Ken Livingstone in the campaign has been the revelation that for years his substantial media earnings were paid into private companies, attracting corporation tax at 20 per cent rather than the higher rates of income tax.

Faced with a constant drip of bad publicity, and having been challenged again today about his tax arrangements, Livingstone went on the attack and accused Johnson of channelling his money through a firm called Finland Station.

That clearly annoyed Johnson, who asserted: "I have never used a company to minimise my tax. There was a TV production company which I was briefly a director of but I have always paid full income tax."