The chief of the Metropolitan Police has come under pressure to explain how Boris Johnson "got off" with just one fine in the Partygate scandal.
Acting Commissioner Sir Stephen House said that did not believe there was "clear evidence" the Prime Minister had breached the rules repeatedly.
Scotland Yard investigated six illegal events Mr Johnson attended and issued fines to others who were present - but the PM escaped further sanction.
However, Sir Stephen insisted his detectives were not influenced by political considerations such as whether they held Mr Johnson's political fate in their hands.
He told the London Assembly police and crime committee: "I don't believe there is clear evidence that the Prime Minister breached many other times."
The senior officer added that police had "not shied" away from issuing fines in Downing Street.
He said: "I'm not particularly concerned about what the PM thinks. I do my job without fear or favour. As did the Met in this situation".
The Mirror revealed last week that Mr Johnson told colleagues he was told by Scotland Yard that he would only be investigated for two lockdown gatherings in Downing Street.
But Sir Stephen said he was "very confident" that the Met gave no such assurances that individuals would escape more fines.
He denied that the political sensitivities of the investigation had prevented the police from issuing more fines for apparent breaches.
But he admitted that some individuals involved in Partygate did not return their questionnaires, although the “vast majority” did.
He claimed Scotland Yard was “not put off” investigating people as a result, adding that if anything it “spurred” them on.
The top cop defended a Downing Street police officer who witnessed the boozy Christmas bash at No 10 but failed to take any action.
"I think it's impossible to expect an officer to walk through a room of people and conclude whether regulations were being breached, when it's taken 12 experienced detectives to work that out," he said.
Scotland Yard has faced criticism for its lack of transparency over Partygate but he insisted the decisions made by detectives were "based on the facts and were proper".
He added: "We looked at whether there was a prescribed exemption under the regulations of the gathering, including whether it was reasonably necessary for work purposes.
"That's an important point - some gatherings we decided were not work-related, and some we decided were work-related."
Sir Stephen said there were "full files of evidence" behind each fixed penalty notice so that if the fine was contested in court there would be a realistic prospect of conviction.
