Boris Johnson lost his cool swearing in to a microphone during a heated debate with opponents at City Hall.

The mayor of London muttered "b******" under his breath after being accused of lying about the impact to the fire service of 20% cuts across the capital.

Bitter exchanges between the Mayor and Labour opponents on the London Assembly were being watched by a noisy contingent of fire service union members. When hecklers accused him of not being straight about how reforms will hit the brigade, Johnson called back "that is not true," before saying "bollocks" to himself.

But the remark was broadcast live to all watching via the microphone on his desk.

The gaffe came soon after Johnson had told one Assembly member to "get stuffed" during the debate.

The former Conservative MP made the remark to Labour member Andrew Dismore, after being told: "You lied to the people of London."

He retracted his remark straight away, saying: "I apologise. It just popped out."

The two verbal gaffes by Johnson come from a media performer whose own sophisticated vocabulary has helped make him a celebrity. Yet Johnson has form on using bad language to make his point.

During the 2012 London election he accused a BBC reporter of talking "f****** b*******" for asking if he was too close to News UK owner Rupert Murdoch.

He also described St Patrick's Day as "lefty crap" during an interview with the New Statesman magazine.