Metropolitan Police Commissioner Cressida Dick told the London Assembly's Police and Crime Committee on Tuesday that officers were looking into potential offenses after being given information by the team investigating a number of alleged parties.
"As a result firstly of information provided by the Cabinet Office inquiry team and secondly my officers' own assessment, I can confirm that the Met is now investigating a number of events that took place at Downing Street and Whitehall in the last two years in relation to potential breaches of Covid-19 regulations," she said.
UK Prime Minister Boris Johnson has been under immense pressure over alleged summer garden parties and Christmas gatherings held in Downing Street when the rest of the country was under strict Covid restrictions.
Johnson's spokesperson said the Prime Minister will cooperate with the police investigation, adding that Johnson does not think he broke the law. "I need to be cautious about what I say but I think that's fair to say that he does not," the spokesperson said Tuesday.
"I welcome the Met's decision to conduct its own investigation," Johnson told Parliament on Tuesday. "Because I believe this will help to get the public the clarity it needs and help to draw a line under matters."
The latest revelation came to light Monday when a Downing Street spokesperson said Johnson celebrated his birthday with a gathering at his official residence in June 2020 while the UK was in its first Covid-19 lockdown.
Under Covid-19 restrictions at the time, indoor gatherings were not allowed, with people permitted only to meet outside in groups of up to six people.
Dick's announcement of an investigation is the latest twist in the weeks-long scandal, which has seen Johnson's approval ratings plunge and growing discontent among some in his ruling Conservative Party, who believe the Prime Minister is becoming a liability.
The Metropolitan Police was criticized for not initially investigating the alleged breaches of Covid laws. Dick defended the force's impartiality on Tuesday, saying "we police without fear or favor. We police impartially and we police in an operationally independent manner."
She however caveated her announcement, saying "the fact that we are now investigating does not, of course, mean that fixed penalty notices will necessarily be issued in every instance and to every person involved."
On Monday, CNN affiliate ITV reported there were two gatherings held to celebrate Johnson's birthday: one attended by as many as 30 people held at the cabinet office the afternoon of June 19, and another that evening that was hosted at Johnson's residence and attended by family friends.
According to ITV News, Johnson's wife, Carrie, organized the small gathering in the Cabinet Room. A source who worked in Downing Street at the time independently confirmed to CNN that the gathering took place on that day and that it was arranged by Carrie, who led the singing of the happy birthday song.
CNN understands that the gathering was largely attended by what was deemed to be Johnson's inner circle at the time.
Downing Street denied that any rules were breached by the evening gathering, with a spokesperson saying: "This is totally untrue. In line with the rules at the time the Prime Minister hosted a small number of family members outside that evening."
The Prime Minister's allies defended him soon after the story broke on Monday.
"So, when people in an office buy a cake in the middle of the afternoon for someone else they are working in the office with and stop for ten minutes to sing happy birthday and then go back to their desks, this is now called a party?" Nadine Dorries, UK's Culture Secretary, asked on Twitter on Monday.
Environment Secretary George Eustice told Sky News: "There was 10 minutes there around sharing a piece of cake, I don't think that really constitutes a party in the way that some of the other more serious allegations being investigated maybe do."
But anger has been brewing about the allegations.
A report into the gatherings and parties, set to be released this week, could be the final straw for Johnson's increasingly mutinous party.
Welsh First Minister Mark Drakeford, who is from the opposition Labour Party, added to the chorus of criticism Tuesday, telling the BBC: "I just don't think the Prime Minister has the moral authority to lead a country like the United Kingdom."
He explained that some of the hardest letters he received over the pandemic were from people "telling me that they had to attend a funeral where only eight people were able to be there; where it didn't last 10 minutes; where you weren't able to sing. In Wales a funeral without being able to sing is a really difficult experience."
Drakeford added "you cannot be someone who asks other people to do things, difficult, upsetting things, that you also...[are] unwilling to do yourself," he said, referring to Johnson.
The deputy leader of the Labour Party welcomed the police's investigation in a statement.
"Millions of people are struggling to pay the bills, but Boris Johnson and his government are too wrapped up in scandal to do anything about it," Angela Rayner said.
"Boris Johnson is a national distraction. Conservative MPs should stop propping him up and he should finally do the decent thing and resign," she added.
The Prime Minister has given unconvincing answers when asked about the numerous parties. First he said there were none. Once undeniable evidence emerged, he denied knowing about the gatherings. When a photo of him at one such event was published, he insisted he didn't realize the gathering was a party, saying he "believed implicitly that this was a work event."
Johnson was even forced to apologize to the Queen after it emerged that a party was held in Downing Street the night before the funeral of Prince Philip. It was noted at the time that because of Covid-19 restrictions, the Queen was forced to mourn her husband at St. George's Chapel at Windsor Castle while sitting alone.