Boris Johnson, the good-time politician with the amiable air of a man who never cracks under pressure, completely lost it yesterday.
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 former mayor in a nose-to-nose confrontation, and called him a "f***ing 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 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 got into a lift to make a 30-second journey for a photo call on the roof. They emerged in front of cameras as if nothing had happened. News of the tantrum leaked out soon afterwards.
London's mayor is one of the sharpest performers in politics, who has got the better of hard-nosed interviewers such as Jeremy Paxman.
Yesterday'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.
Halfway through, it was showing Mr Johnson and Mr Livingstone in front on 25pc each, but, at the end of the hour, Mr Livingstone and Mr Paddick were level on 24pc while Mr Johnson was trailing on 16pc.
It should be said in Mr Johnson's defence that very few politicians can match Mr Livingstone's special talent for driving others to fury.
Mr 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 Mr Livingstone in the campaign has been the revelation that, for years, his substantial media earnings were paid into private companies, attracting corporation tax at 20pc rather than the higher rates of income tax.
Having been challenged again yesterday about his tax arrangements, Mr Livingstone went on the attack and accused Mr Johnson of channelling his money through a firm called Finland Station.
That clearly annoyed Mr 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."
Mr Johnson is very highly paid. There is his £145,000 (€174,000) salary plus £250,000 for his 'Daily Telegraph' column. If he pays income tax on the lot, the sum he hands over to Inland Revenue must be enormous. To be accused of cheating the tax man was evidently too much, even for one of the coolest men in politics.