ITV reported on Monday that Johnson and his now wife Carrie had attended a surprise party of up to 30 people for his birthday in the Cabinet Room at No. 10 Downing Street on June 19, 2020, when indoor gatherings were banned.

Johnson’s office described the alleged party as a brief gathering by staff to wish him happy birthday, adding that he was there for “less than 10 minutes”. A spokesman said Johnson did not believe he had broken the law over any of the gatherings.

“I welcome the Met’s [London Metropolitan Police’s] decision to conduct its own investigation because I believe this will help to give the public the clarity it needs and help to draw a line under matters,” Johnson told parliament.

Met Commissioner Cressida Dick, Britain’s top police officer, said an investigation had been opened into a number of events “at Downing Street and Whitehall in the last two years”. Police had initially refused to look at the gatherings.

The Cabinet Office said its own investigation, by senior official Sue Gray, was continuing and there was ongoing contact with the police.

Johnson’s spokesman said Gray’s team was discussing with the police whether she could still release her report in full before their investigation had been completed.

“I have seen reports suggesting we were seeking to prevent the publication and that is not accurate,” the spokesman said. Anyone who is asked to will cooperate fully with the investigation, he said.

Reports of the gatherings have seen Johnson’s ratings plunge, with much of the public and some of his 359 Conservative Party lawmakers calling for him to resign. So far, there are fewer than the 54 lawmakers required to trigger a confidence vote that could result in a leadership contest, but patience is wearing thin.

“With the police now investigating, this nightmare gets even worse,” senior Conservative David Davis, who has already called on Johnson to quit, said on Twitter.

The police said they had no time frame for the investigation and government minister Michael Ellis told parliament: “We have no idea how long that [investigation] will be.”

Johnson has survived scandals throughout his career but his premiership, straddling both Britain’s departure from the European Union and the worst pandemic for a century, has been defined by turbulence.

His 2019 plan to suspend parliament and force Brexit through was overturned by the Supreme Court. Eventually, to the delight of millions who changed their political allegiance to vote for him, he negotiated a divorce deal with the EU.

When the pandemic struck, he drew criticism for delaying shutting the country down. He nearly died from Covid-19 himself before recovering to oversee a world-leading vaccine roll-out.

But reports that he and his staff were flouting the rules they imposed on the British public are testing Johnson’s legendary ability to bounce back.

“You break the rules, you’ve got to go,” said Ian Dowrich, 59, a builder from Brentwood, Essex, who said he had voted for Johnson.

Police chief Dick said police had not typically investigated every alleged lockdown breach but she felt there were grounds to do so now after receiving some findings from Gray’s inquiry.

Deputy Leader of the Labour Party Angela Rayner welcomed the investigation and renewed calls for Johnson to resign. “Boris Johnson is a national distraction,” she said.