Boris Johnson is facing fresh questions after photos showing the prime minister drinking at an event during a Covid lockdown were published.
The pictures, released by ITV News, are believed to show the prime minister at a leaving party on 13 November 2020.
The BBC has been told that at least one person who attended the event was fined, but the PM was not.
The Metropolitan Police are facing calls to explain why Mr Johnson did not receive a fine.
A government source told BBC News the photos may have been taken by the official No 10 photographer and proved Mr Johnson was there in a work capacity.
The police investigated two events in Downing Street on 13 November. Asked in Parliament by a Labour MP whether a party had taken place on that date, Mr Johnson said "no", adding that "I'm sure...all the rules were followed".
Meanwhile, a government source has also told the BBC that civil servant Sue Gray may hand her report on parties to Downing Street on Wednesday, followed by a statement in Parliament and a press conference by the prime minister.
Responding to the leaks, Labour's Jonathan Ashworth said it would be difficult for the PM to reconcile his statement to Parliament that no party had taken place with the pictures and called on Tory MPs to remove him from office.
The peer and former leader of the Scottish Conservatives, Ruth Davidson, said the prime minister had lied to Parliament and his position was now untenable.
Senior Conservative Tom Tugendhat was also critical telling the BBC that: "Seriousness in government matters. It costs us all. And I'm afraid this just doesn't look serious."
Former justice secretary Robert Buckland told BBC Radio Wiltshire that: "If there's a deliberate lie, I can't see how anybody, including this prime minister, can continue."
"There are things we say honestly and genuinely at the time that we believe to be true... now that's one thing. Going and deliberately saying X is Y knowing that is the case is, of course, beyond the pale."
But Transport Secretary Grant Shapps said that while he found the picture "difficult to look at", he suggested the presence of the PM's ministerial red box "suggests he was probably on his way through".
"I think he's popped down there to raise a glass and say thank you to a long term member of staff who is leaving," he said, adding that the police would have "thoroughly investigated" the event.
He also said the prime minister was "mortified".
A No 10 spokeswoman said the prime minister would address Parliament "in full" after senior civil servant Sue Gray published her report into the gatherings "in the coming days".
There was support for the PM from Conservative backbench MP Sir Desmond Swayne, who said he believed it was a "work do", adding "that's what people do at work".
He told BBC Newsnight: "Now I understand entirely the public anguish at a time when they were locked down and they were not supposed to be together, but there clearly was a distinction between the workplace - where people work together and are effectively in a bubble - and what was taking place outside."
How much do the Partygate photos matter?
Just when the prime minister might have hoped the bumpy moments of Partygate might be beginning to ease, these pictures emerge.
We already knew about the event in question.
But there's one thing knowing about a party, something else seeing pictures from it. Pictures of partying when the rest of the country was in lockdown.
The big question, though, is how much does it matter?
How many minds will move, particularly among Conservative MPs who, remember, have the capacity to remove a party leader if they choose?
You can read Chris's full blog here.
The prime minister and the Met are under fresh scrutiny after ITV News published four new photographs on Monday which it says were taken at the leaving do for Mr Johnson's communications chief, Lee Cain on 13 November 2020.
Mr Johnson is pictured toasting colleagues while standing by a table laden with wine bottles, wine glasses, food and other drinks.
A second coronavirus lockdown was in place in England at the time the photographs were taken, with indoor gatherings of two or more people banned, except if "reasonably necessary" for work purposes.
Allegations about gatherings held in Downing Street and Whitehall throughout the pandemic led to a Metropolitan Police investigation into 12 events.
The inquiry concluded last week, with a total of 126 fixed penalty notices being issued to 83 individuals for parties held over eight separate dates.
Mr Johnson, his wife Carrie, and Chancellor Rishi Sunak all received one fine each for attending a birthday party thrown in the PM's honour in June 2020.
But No 10 confirmed the PM was not facing any additional fines from the police.
The emergence of the photographs has sparked fresh claims from opposition MPs and others that Mr Johnson knowingly misled Parliament when he previously told them no Covid rules had been broken in Downing Street.
On 8 December 2021, Labour MP Catherine West asked Mr Johnson in the Commons if a party had taken place in Downing Street on 13 November 2020 - though it is not clear if she was referring to the event photographed.
The PM replied: "No, but I'm sure whatever happened the guidance was followed and all the rules were followed at all times."
The prime minister faces a probe by the Commons' Privileges Committee about whether he lied to MPs. Under government guidelines, ministers who knowingly mislead Parliament are expected to resign.
MPs are also awaiting the outcome of an investigation by Durham Police into a gathering in the city on 30 April 2021 attended by Labour leader Keir Starmer and his deputy Ms Rayner.
Sir Keir has said he will resign if he is fined for attending the event.
Meanwhile, questioned about its decision making, the Met has declined to explain why the prime minister was not fined over the leaving party.
The Liberal Democrats have written to the Independent Office for Police Conduct, urging it to investigate the Met's probe into events in No 10 and Whitehall during lockdown.
The party's deputy leader Daisy Cooper said: "If anyone else had been pictured at a party like this during lockdown, surely this would have been enough evidence for them to be fined."
The Labour London Mayor Sadiq Khan said he though the police should "explain why they have reached their conclusions and provide that clarity".
A No 10 spokeswoman said the Cabinet Office and the police had been given access to information, including photographs.