The Prime Minister, who will today face the Commons at PMQs, said he welcomed Scotland Yard’s investigation and hoped officers would “help to draw a line under matters” after his leadership was plunged into deeper jeopardy by the development on Tuesday.

Downing Street signalled he is willing to speak to the officers investigating alleged breaches of coronavirus rules over the past two years, but said Mr Johnson believes he has not broken the law.

Downing Street parties: As the Met Police launch an investigation into partygate...

Scotland Yard has launched an investigation into a "number of events" in Downing Street and Whitehall

Uncertainty was cast over the publication of Ms Gray’s inquiry into claims of lockdown breaches, but it was understood her report could still be published in the coming days as her team hold discussions with police.

A No 10 source said they were yet to be told when Mr Johnson would receive the report, amid suggestions it could come as soon as Tuesday night, in advance of its wider publication.

Downing Street initially suggested that elements of the long-awaited Cabinet Office inquiry that touch on potentially criminal acts may be paused now police are involved.

But after it emerged Scotland Yard had not objected to any publication, No 10 said it was not trying to block the report and said Ms Gray’s team were in talks over “what may or may not be published”.

Boris Johnson speaks to MPs to update them on the situation in Ukraine on Tuesday

Conservative MP Andrew Bridgen, who has called for the Prime Minister to resign over the partygate scandal, told Channel 4 News he hoped Ms Gray “gets her report in tonight and it is published tomorrow”, arguing the affair was “paralysing the Government”.

Commissioner Dame Cressida Dick said that officers are investigating a “number of events” in Downing Street and Whitehall, after being passed information from the Gray inquiry.

Mr Johnson told the Commons that he welcomes the Met investigation “because I believe this will help to give the public the clarity it needs and help to draw a line under matters”.

The Prime Minister’s official spokesman said that “anyone asked to will co-operate fully” when asked if Mr Johnson is willing to be interviewed by officers.

Pressed on whether the Prime Minister thinks he has not broken the law, the spokesman said: “I need to be cautious about what I say but I think that’s fair to say that he does not.”

Labour leader Sir Keir Starmer said the Gray report must be published “in full” so there is no “cover-up”.

Mr Johnson’s spokesman initially said the inquiry “won’t publish anything that relates to the work of the police”, suggesting only elements that do not relate to possible criminal acts may be released.

But the PA news agency understood Scotland Yard had not objected to the publication of any part of the Gray report.

Sources close to the investigation suggested that Ms Gray was considering her options and was concerned about publishing a report which was shorn of some of its key findings.

And they said that while the format of the publication was still unknown, the report could still come this week.

Downing Street updated its position in the afternoon to acknowledge that discussions are “still ongoing between the investigations team and the police” including on “what may or may not be published”.

Some Tory MPs have publicly called for Mr Johnson’s resignation, but others have said they will await the publication of the Gray report before trying to trigger a vote of no confidence.

The decision to open a police investigation was taken as a result of information from the Gray inquiry and “my officers’ own assessment”, Dame Cressida said, when announcing the development to the London Assembly’s Police and Crime Committee.

She pledged to only give updates at “significant points” and declined to say which alleged parties are under investigation, nor would she put a timeline on when officers could detail their findings.

The commissioner said investigations are carried out into “the most serious and flagrant type of breach” where individuals knew they were committing an offence or “ought to have known”.

Deputy Assistant Commissioner Jane Connors, the Met’s Covid-19 lead, will oversee the investigation being carried out by the special inquiry team, which works on sensitive and confidential work involving high-profile subjects and offences by those in public office.

The Met had been under pressure to open an investigation for weeks, with the Daily Mirror first reporting allegations of parties in No 10 during Covid restrictions two months ago.

Fresh allegations have emerged at a steady pace since then and have now totalled at least 19 separate events.

The latest emerged on Monday when Downing Street was forced to admit Mr Johnson had a birthday celebration inside No 10 during the first lockdown.

Downing Street conceded staff “gathered briefly” in the Cabinet Room following a meeting, after it was alleged 30 people attended and shared cake despite social mixing indoors being banned.

ITV News reported the Prime Minister’s wife, Carrie Johnson, had organised the surprise get-together complete with a chorus of “happy birthday” on the afternoon of June 19 2020.

Northern Ireland minister Conor Burns, an ally of Mr Johnson, told Channel 4 the Prime Minister had been “ambushed with a cake”, adding: “I don’t think most people looking at that at home would characterise that as a party.”

Chancellor Rishi Sunak was understood to have briefly attended as the gathering was breaking up, as he entered the room to attend a Covid strategy meeting.

ITV reported picnic food from M&S was eaten and Martin Reynolds, Mr Johnson’s under-fire principal private secretary, was also said to have attended, as was No 10’s director of communications Jack Doyle and head of operations Shelley Williams-Walker.

Social gatherings indoors were forbidden under lockdown laws at the time, with a relaxation of the regulations permitting gatherings of up to six people to take place outside.

Downing Street partygate: Boris Johnson faces critical day in battle for survival ahead of Sue Gray report publication