Scotland Yard is under pressure to explain why the Prime Minister was not fined for attending a partygate gathering in Downing St, as photographs emerged showing him raising a toast with a group of colleagues.
It comes as Sue Gray's report into the events in Number 10 during the Covid crisis could be published as soon as Tuesday.
The pictures were taken at a leaving party for Lee Cain, Boris Johnson's former director of communications, on November 13, 2020, when the country was in a second lockdown.
In the photographs, the Prime Minister can be seen holding a glass of what appears to be Champagne with a group of about seven other people, who are all standing in close proximity to one another.
It is understood that while at least one other attendee at the party received a fixed penalty notice, Johnson never received a questionnaire asking him to explain his presence.
The Prime Minister had been keen to move on from partygate and focus his attention on the cost of living crisis. He had hoped to put it behind him when the Metropolitan Police completed its investigation and it emerged he had been issued with only one fine.
But Johnson is now facing fresh questions over whether he misled parliament about the events of November 13, having told MPs that he was "sure that whatever happened, the guidance was followed and the rules were followed at all times".
He is also under pressure from members of his own party to explain what happened and to reconcile his account with the photographs.
The images were published by ITV News amid speculation that Dominic Cummings, the Prime Minister's disgruntled former aide, may have been the source.
It is understood that some of those who attended the event are angry that the Prime Minister has not been fined.
A source with knowledge of the party said: "I can't work out the reason why he's not been fined and other people have.
"The Met's a shambles. They've been told a lot of stuff and they've just ignored it.
"It's so obviously a breach. Whatever way you look at it, it's a breach. None of us can figure it out."
The Liberal Democrats have written to the Independent Office for Police Conduct asking for an investigation into the Metropolitan Police's handling of the allegations.
Lord Paddick, a former deputy assistant commissioner of the force, said: "It appears the evidence from the photograph proves beyond all reasonable doubt that the Covid regulations have been breached and therefore why the police have not issued a fixed penalty notice makes no sense."
Metropolitan Police sources said they were concerned about the fallout from the investigation and that their failure to be transparent about decision-making would further damage the force's already tarnished reputation.
Senior Scotland Yard officers have refused to explain how detectives came to their decisions. However, Sir Stephen House, the Acting Commissioner, is expected to face questions when he appears before the London Assembly Police and Crime Committee on Thursday.
Marina Ahmed, who is a member of the committee, said: "The photos that have emerged raise significant questions for the Metropolitan Police over whether they had access to this evidence, and if so, how investigators arrived at their judgment not to issue Boris Johnson with a fixed penalty notice for the event on 13 Nov 2020."
Johnson is also facing renewed criticism in the wake of the emergence of the photographs.
Angela Rayner, the deputy leader of the Labour Party, tweeted:
Rayner is herself under investigation by Durham police over the beergate event, along with Sir Keir Starmer.
Parliament's Privileges Committee has already been asked to investigate whether Johnson intentionally misled the Commons in statements denying that Covid rules or guidance were broken during lockdown in Downing St.
Chris Bryant, who normally chairs the committee but has recused himself from this investigation, said the emergence of photographs made it more likely the committee would find against him.
"I think people thought it might be difficult to prove, but it's looking a bit easier now," he told the Telegraph.
On Monday, the Prime Minister was also facing questions from Tories.
Douglas Ross, leader of the Conservative Party in Scotland, said Johnson "must outline why he believes this behaviour was acceptable" and described the photographs as "unjustifiable and wrong".
Roger Gale, the Tory MP who has previously called for Johnson to resign, said: "It's absolutely clear there was a party, that he attended it, that he was raising a toast to glass one of his colleagues. Therefore, he misled us from the despatch box. Honourably, there is one answer."
Other Conservative MPs suggested they may break ranks and attack Johnson over partygate once again, after months of relative stability on the Tory backbenches.
Steve Baker, an influential MP on the Tory Right, tweeted a pandemic-era government advert about the Covid rules:
Just hours earlier before the images emerged, Cummings wrote on his blog that photographs would soon emerge that would show that Johnson had "obviously lied to the Commons and possibly to the cops".
Cummings has previously been accused of leaking damaging material about Johnson since leaving Downing St in November 2020.
Scotland Yard announced that it had concluded its four-month partygate investigation and had issued a total of 126 fixed penalty notices to 83 people, linked to events on eight separate dates.
In April, Johnson, his wife Carrie and Rishi Sunak, the Chancellor, were all fined for attending a nine-minute birthday party in Downing St in June 2020, when the restrictions were more relaxed.
But despite speculation that Johnson was in line to receive further fines, he escaped any additional sanction.
Sources close to Gray said she had always considered the Lee Cain leaving party to be the most egregious of all the events Johnson attended.
She was said to be "flabbergasted" at the decision by the Metropolitan Police not to fine Johnson over the gathering.
A spokesman for Gray refused to comment on whether the photographs of Johnson and others at the leaving do were among the 510 images collected by her investigation and examined by the Met.
Downing St sources suggested the reason Johnson was not fined over the leaving party may be because he had only briefly attended it on his way from his office to the private flat he lives in above Number 11.
It is understood the event had already started when the Prime Minister arrived and gave a speech.
Cummings and Cain did not respond to a request for comment.