Boris Johnson has repeatedly accused Ken Livingstone of being a "f***ing liar" in a post-hustings clash in a lift.
The incumbent London mayor's temper reportedly boiled over in a crowded lift after a live hustings event at the LBC 97.3 radio station this morning.
ITV cited sources reporting that Boris' face was "red with rage" after their "nose-to-nose" clash. The Conservative was angry over Ken's claims that they had the same arrangements for declaring income tax.
Mr Johnson was quoted as telling his main challenger: "It's all f***ing lies, it's all f***ing lies."
Another version quoted him as saying: "You're a f***ing liar, you're a f***ing liar, you're a f***ing liar."
Liberal Democrat candidate Brian Paddick, the Greens' Jenny Jones and LBC's managing editor James Rea were also in the lift as they travelled to a roof terrace photo call.
"As an incumbent who has built a brand based on being upbeat, his campaign's relentless negatives on Ken Livingstone are damaging to the carefully constructed Boris Johnson image," a spokesperson for Mr Livingstone said.
"Unlike four years ago the Tory candidate is now subject to real scrutiny by the other candidates - and his touchy reactions indicate that as an establishment figure he is not used to it."
In a statement issued afterwards, Boris said: "In relation to my business affairs and tax arrangements, specifically do I have any company or other arrangements constructed to enable me to pay less tax and do I, as has been claimed by the Labour mayoral candidate and the opposition leader, have the same arrangements as Labour's mayoral candidate, the answer is simple and unequivocal in both cases: no."
Labour wants to know whether Boris paid income tax on all of his earnings from Finland Station, a TV production company of which he was a director and 33% shareholder.
It is also questioning whether he paid capital gains tax at 18% rather than income tax at 40%.
Questions about Boris' tax arrangements follow incessant claims about tax avoidance by Ken, who allegedly used his Silveta company to pay corporation tax at 20% instead of the higher rates of income tax.
Mr Livingstone was quoted as saying earlier this year: "These rich bastards just don't get it… no one should be allowed to vote in a British election, let alone sit in parliament, unless they pay their full share of tax."