The two men were travelling in a lift up to be photographed after they took part in a live mayoral debate on a radio show at LBC 97.3 when BoJo unleashed his fury.

After Livingstone claimed on air that the Conservative Mayor was avoiding tax through a private company, Johnson shouted: “You’re a f***ing liar! You’re a f***ing liar! You’re a f***ing liar!”

Sources claimed that Johnson was “visibly shaking with anger”, “red with rage” and “right up in Mr Livingstone’s face” when he started swearing.

Liberal Democrat mayoral candidate Brian Paddick and Green party candidate Jenny Jones were in the lift at the time, along with LBC managing editor James Rea.

Livingstone’s team confirmed to the Guardian that Johnson had called him a “f***ing liar”.

A spokesman 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.”

Livingstone has recently come under fire for “tax dodging” over claims that he was paid through a company so he was only liable for 20 per cent corporation tax rather than income tax.

Livingstone accused BoJo of having “the same arrangements” for his media earnings.

On the radio show Johnson said: “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.”