Mr Johnson said police handling of the matter was "disproportionate"
London's mayor has been accused of abusing his political position after admitting contact with a senior Tory MP currently under police investigation.
Boris Johnson told the London Assembly he spoke with Tory immigration spokesman Damian Green following his arrest into alleged Home Office leaks.
Mr Johnson, as chairman of the Metropolitan Police Authority (MPA), was briefed on the arrest by the Met.
Critics said it was "inappropriate" for the mayor to talk to a police suspect.
Mr Green was arrested and held for nine hours last Thursday on suspicion of conspiring to commit misconduct in public office and on suspicion of aiding and abetting misconduct in public office.
All his homes and offices were searched. Mr Green denies any wrongdoing.
Speaking to the London Assembly on Wednesday, Mr Johnson said: "I think this particular police action could easily be construed as disproportionate.
"I have yet to be persuaded that it's likely to yield a charge or successful prosecution.
"And that was why I spoke in advance as I did and I think I was absolutely right to do so."
But his admission was seized upon by Labour opposition members at the London Assembly who accused him of behaving "inappropriately".
London Assembly Labour group leader Len Duvall said: "It is astonishing that the mayor, following a briefing from senior police officers, has been speaking to a suspect under police investigation.
"He received information as chair of the MPA, then went to his political ally and old friend, who is now under criminal investigation.
"This is not appropriate behaviour for a chair of the police authority."
He added: "Whether he likes it or not, the public perception will be that he is too involved in this investigation and is looking after his mates."