The Conservative and Labour politicians told one another to "man up" as they entered into a heated exchange about the UK's relationship in the EU.
Johnson said that it is "very sad that we are being invited to continue to remain in a system that is less and less democratic" but all you hear from the Remain campaign is negativity about our chances outside the EU.
He added that Umunna had been quoted as saying 50 per cent of law in the UK comes from the EU.
But Umunna hit back, saying the mayor had got that wrong. "The House of Commons library says that around 13 per cent of our laws come from the EU," he said.
So Johnson told Umunna to "man up" and admit he had used the 50 per cent figure, prompting the Streatham MP to retort that Johnson should "man up" and stop seeking to put words in his mouth.
Some agreement seemed to have been inadvertently found as Johnson and Umunna moved to tell each other that the campaign wasn't about them, and was about the prospects of the British people and policy.
Talking about the prospects of a Canada-style trading deal, the mayor said the EU would not put tariffs up against the UK and that "the sheer negativity and lack of optimism about the ability rises off the Remain campaign like a vapour".
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But Umunna said if anyone has been negative about the UK it had been Johnson. "You denigrate our influence in the EU ... I disagree with Michael Gove on a huge amount, but he is a man of conviction. You brought a circus to your house to make the announcement of which way you were going to campaign."
The exchange comes as the campaign moves into its last 100 days, with polls still relatively close. And earlier today Prime Minister David Cameron's election guru Sir Lynton Crosby said the race was still open.
Johnson also hinted that the rival Leave campaigns could unite. He said the "basic objective" should be to have a single campaign and the rival groups were "unified by the rightness of their arguments".