Boris Johnson is battling fresh allegations that he “instigated” a Downing Street party during Covid lockdown restrictions.
Tory rebels have also warned that the Prime Minister could face a new coup attempt after May’s local elections, with the Conservatives expected to perform badly in the wake of “partygate”.
It has been claimed a photographer was present at the gathering on November 13 2020 to mark the exit of Lee Cain, Number 10’s former director of communications.
The photographer reportedly captured images of Mr Johnson, who allegedly delivered a speech, poured drinks for people and was drinking himself.
“This wasn’t a leaving drinks - until the Prime Minister arrived,” a source told The Sunday Times.
“This was the usual press office Friday evening wash-up drinks. Boris came fumbling over, red box in tow, and he gathered the staff around the press office table, which did have bottles of alcohol on it.
“He said he wanted to say a few words for Lee and started pouring drinks for people and drinking himself. He toasted him.
“It was in Downing Street, people around the table, and there was loads of alcohol. If you put that picture on the front pages, I think a lot of people, MPs, who have priced a lot of stuff in will not have priced that in, because it looks like a massive p——.”
Downing Street did not dispute the description of the event, but said it was untrue that Mr Johnson had organised it.
At the time, organisers of large gatherings faced penalties of up to ￡10,000.
Mr Johnson is expected to receive a second fine from the Metropolitan Police, having already been penalised for attending a gathering in the Cabinet Room to mark his 56th birthday.
His alleged actions at the gathering to mark Mr Cain’s departure - which took place on the same evening of the so-called “Abba-themed” party in the Downing Street flat to celebrate Dominic Cummings’ departure - threaten to derail his argument that “all rules were followed”.
Asked in the Commons whether there had been a party in Downing Street on Nov 13, 2020, he said: "No, but I am sure that whatever happened, the guidance was followed and the rules were followed at all times."
Mr Johnson now faces one of the toughest political weeks of his premiership, as he prepares to make a statement in the House of Commons on Tuesday over his partygate fine.
It is expected that he will explain the wider context of his “birthday party” on June 19 2020 and apologise for breaking the rules, but insist he was not aware he had done so. He will also urge MPs to retain a sense of perspective amid the Ukraine crisis.
On the same day, Sir Lindsay Hoyle, the Speaker of the House of Commons, is expected to allow MPs a vote on an investigation into whether Mr Johnson misled the Commons over the extent of lockdown parties in Downing Street and across Whitehall.
On Wednesday, MPs will then vote on whether the matter should be handed over to the privileges committee - a seven-strong group of parliamentarians who would investigate whether the Prime Minister is guilty of wrongdoing.
The committee - which has a Tory majority but is chaired by Chris Bryant, a Labour MP – would rule on whether Mr Johnson has been found in contempt of Parliament. If he is, he could be suspended from the House of Commons.
Conservative backbenchers privately fear "a week none of us are looking forward to", as the opposition motions force the debate around the Prime Minister's future to resurface.
One MP suggested it would be a mistake for the Tory whips to intervene if the votes took place, likening it to the Owen Paterson debacle and a dramatic about-turn ahead of the vote on publishing security advice about Lord Lebedev last month.
The MP admitted that very few backbenchers, if any, would be willing to give Sir Keir Starmer, the Labour leader, a political victory by supporting a motion that condemned their own party leader.
"At the moment, we are taking away every single possibility of people feeling that they should vote for us," the MP added. "We are making people feel dirty for being Conservative - and that is what is hurting us."
Labour has accused the Prime Minister of “demeaning his office”. Angela Rayner MP, the party’s deputy leader, said: “If the latest reports are true, it would mean that not only did the Prime Minister attend parties, but he had a hand in instigating at least one of them. He has deliberately misled the British people at every turn.”
One former Conservative minister told The Telegraph that no-one in the party wants to “rock the boat” ahead of the May elections - but warned that if the Tories fare badly, it could be a “catalyst” for renewed attempts to oust their leader.
“There are lots of individuals who are rather upset to say the least, but I’m not aware that there is a new organised move to try and unseat him,” he said.
“I think we are going to have a tough enough time as it is, to add anything else to rock the boat with three weeks to the election.
"I think a lot of Tory MPs would have liked to have moved on but it has become such a big issue that they can’t. The more fixed penalty notices and the more instances that come out, it doesn't make it any easier for him.”
However, some on the Tory benches sounded a more optimistic note. One loyalist of Mr Johnson insisted there was a "lot of support" for the Prime Minister on the doorstep during local campaigning.
Steve Baker, the Conservative MP, added that it is “fanciful” to think Tory MPs would “move against the PM this close to a council polling day”, adding: “Whoever is briefing against the PM plainly doesn't understand how MPs work.”
Peter Hennessy, a political historian who sits in the Lords as a crossbench peer, told the BBC on Sunday: "The Queen’s first minister is now beyond doubt a rogue prime minister, unworthy of her, her Parliament, her people and her Kingdom.
"I cannot remember a day when I’ve been more fearful for the wellbeing of the constitution.”