There is no “clear evidence” that Boris Johnson breached lockdown regulations more than once, according to the Metropolitan Police.

Acting Met Commissioner Sir Stephen House was grilled by members of the London Assembly on Thursday afternoon following the conclusion of Scotland Yard’s investigation into “partygate”.

The Met has come under increasing pressure to justify why the Prime Minister was not fined for attending a party during lockdown after pictures emerged this week that showed him at a booze-filled gathering which was supposedly a leaving do for aide Lee Caine.

Speaking on Thursday, Sir Stephen House refused to answer a question from Labour’s Unmesh Desai about whether the Prime Minister had indeed been issued a Fixed Penalty Notice for attending that gathering in November 2020, despite the Met having already confirmed that he had not.

Operation Hillman – the Met’s formal investigation into Partygate – resulted in just one Fixed Penalty Notice for Boris Johnson over a party he attended in the Cabinet Room in Downing Street in June 2020.

Last week, Scotland Yard confirmed the Prime Minister would not receive any more fines after concluding its investigation, just days before the new pictures emerged.

Speaking at City Hall, Sir Stephen House said: “I don’t believe there is clear evidence that the Prime Minister breached [regulations] many other times.

“I am very confident of the integrity of the decisions that were made in this investigation, which was a difficult investigation. I would like to pay tribute publicly to the officers who were involved in this investigation.”

Sir Stephen, who is acting as Met Commissioner until a permanent replacement for Cressida Dick is appointed, said he accepts that “many of the photographs [of Downing Street parties] look bad”, but added “we deal with the law, not what looks bad”.

He said: “Just because there is alcohol present, can I just remind people that [the investigation] is about breaching Covid regulations, it’s not about whether there was drink there or not. We have to put Fixed Penalty Notices to people that we think will win in court.”

The acting Commissioner went on to say that work-related events were a reasonable justification for groups of more than two people to gather during lockdown, and that “a photograph can be somewhat deceptive”.

Mayor of London Sadiq Khan was one of many politicians who requested the Met explain its decision not to fine the Prime Minister after the new photographs emerged.

In a letter published on Thursday afternoon in response to Mr Khan’s questioning, Sir Stephen House said: “I understand the public have many questions and I have tried to explain as clearly and openly as I can the factors that led to the decisions we took. I can assure you and the public that the small but skilled team investigating this have acted diligently, proportionately, carefully and impartially.”