An agitated Boris Johnson ordered Barnet GLA member Andrew Dismore to “stop endlessly attacking councillors' reputations” during a meeting of the London Assembly.
The outburst came after Mr Dismore told Mr Johnson about how Barnet Mayor Hugh Rayner is under investigation for acting “illegally” towards tenants living in the properties he owns.
In addition to charging above market rates, Cllr Rayner failed to declare his interest in housing during council meetings and did not join the London Landlord Accreditation Scheme.
At last week’s assembly meeting, Mr Dismore told Mr Johnson that Conservative councillors Peter Zinkin, Dean Cohen, Melvin Cohen, Helena Hart and Tom Davey also failed to join the non-compulsory scheme.
He explained how at the last full council meeting, the six in question secured a dispensation from the council to disapply the normal pecuniary interests rule to vote on a housing issue.
Mr Dismore, who did not attend the full council meeting in question, said: “Do you think that’s right?”
But with Mr Dismore having already raised the issue about landlord councillors three times in the space of a month, Mr Johnson was less than pleased.
He said: “If I may respectfully say to you, you’ve raised the gentleman concerned several times in this place and raised a series of allegations about it.
“If you feel there is some evidence of criminal misconduct, you must take this up with the police.”
Mr Dismore, who is standing for MP in the 2015 elections, also outlined how more than 160 firms in Premier House, Edgware, have been served with eviction notices from their premises so they can be turned into flats.
Barnet Borough Council agreed a change of use request after new laws introduced last year left it unable to refuse Reichmann Properties' appeal to turn the block into flats.
After discussing the issue, Mr Dismore invited Mr Johnson to meet with Barnet Council to offer help.
Mr Johnson added: “I wouldn’t want to interfere with your relationship with Barnet Council, which is very close and strong.
“I encourage you, Andrew, to do your job, rather than endlessly trying to attack the reputations of people in this place who aren’t members and have no ability to defend themselves.
“You have now several times used your position as an elected member to do this.
“I know everyone rejoices in how you are a member of this assembly, and they will even more rejoice at the prospect of your departure from it to be an MP.”