Boris Johnson accused Ken Livingstone of being a "f*****g liar" as a furious row over the mayoral candidates tax arrangements spilled off air following a radio debate.
The Labour candidate and former mayor has been accused of deliberately funneling some of his earnings into a company in order to, legally, pay corporation tax of 20% rather than income rate of 50%.
In a bad tempered exchange he accused Johnson of having had a similar arrangement in the past through a company called "Finland Station".
However Johnson denied the charge, insisting that he had never used a company to minimise the amount of income tax he paid.
"I pay income tax, I'm proud of it, it's pretty disgraceful to be attacked by Ken Livingstone, someone who set up a dodging maneuver then denounced everybody else who did so," he said.
Johnson said the company was a television production company that he was "briefly a director of" before he came mayor. "The guy's a liar," Johnson muttered as Ken continued his attack, "a barefaced liar".
The fight reportedly continued into the lift at the radio station as Livingstone, Johnson and the Lib Dem candidate Brian Paddick and the Green candidate Jenny Jones left the studio.
According to The Guardian Johnson confronted Livingstone over his accusations, saying to him: "You're a f*****g liar, you're a f*****g liar, you're a f*****g liar."
In a statement published following the debate, Johnson said: "No income earned by me has ever been paid to a "service" company, through which a person or person's freelance earnings can be channelled so that they pay corporation rather than income tax.
"To suggest otherwise is a complete and utter fabrication.
He added: "Of course the real point is not about my tax arrangements. It is about the hypocrisy of a man who for years has railed against those who use special arrangements to reduce their tax and who has then been caught – bang to rights – doing the very same thing himself."
Following today's debate, a spokesperson for Livingstone said: "Boris Johnson lost his temper because he lost the debate - he talked about cable cars not cutting fares and the squeeze on Londoners."