Ken Livingstone and Boris Johnson clashed furiously over their tax arrangements in the first live debate ahead of the mayoral elections.
Mr Johnson denied suggestions he had used a company to channel his media earnings to avoid paying income tax - a claim that he has levelled at his Labour opponent.
Mr Livingstone accused the mayor of having "the same arrangements" for dealing with his media earnings as he had, insisting he had used a company called Finland Station.
Mr Johnson called the claim "lies", adding: "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 certainly never ... I have always paid full income tax."
More top news
MI5 face mounting pressure over claims the IS executioner 'Jihadi John', named yesterday as a London graduate, was known to them
Labour leader Ed Miliband is due to set out his party's plans on how it would cut tuition fees.
New drug driving laws are to come into effect next week, including laws on some prescribed medications.