Johnson is fighting for his political life after allegations that he and staff partied at the heart of the British government in breach of rules they had themselves imposed to fight the coronavirus pandemic.
Revelations of revelry including boozy parties in Downing Street, suitcases of supermarket alcohol, a broken children's swing, a wine fridge and jokes by staff about how to present such parties to reporters, have hammered Johnson's ratings.
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 a group of staff to wish happy birthday to the prime minister, who 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) 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, the United Kingdom'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 co-operate fully with the investigation, he said.
Reports of the series of gatherings have prompted some of Johnson's 359 Conservative Party MPs to call for him to resign although so far fewer than the 54 required to trigger a confidence vote that could result in a leadership contest.
Some had indicated they would wait for Gray's report 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.
"We have to be able to get back to dealing with real threats as quickly as possible."
The police said they had no timeframe for the investigation and government minister Michael Ellis told parliament: "We have no idea how long that (investigation) will be."
"I would urge the house to wait for the findings of (the Cabinet Office) investigation and for the police to conclude their work," he said.
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.
She said police had looked at several factors when deciding whether to investigate any reported COVID-19 rule-breaking, including if "there was evidence that those involved knew or ought to have known that what they were doing was an offence".
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.
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