Prime Minister Boris Johnson has apologised for attending a "bring your own booze" party during the first coronavirus lockdown.
He told MPs the event in the Downing Street garden was "technically within the rules" but he should have realised how it would look to the public.
Labour leader Sir Keir Starmer said the PM must now quit over what he called his "ridiculous" lies and excuses.
Mr Johnson is also under pressure from his own MPs over the May 2020 party.
After Prime Minister's Questions, he toured the Commons tea rooms, where MPs gather, to shore up support among his backbenchers.
If 54 of them send letters to the 1922 committee - the influential backbench group which runs Tory leadership contests - it will trigger a challenge.
Scottish Conservative leader Douglas Ross, an MP and MSP, said he would be writing to the committee because he believed the PM's position was "no longer tenable".
"He is the prime minister, it is his government that put these rules in place, and he has to be held to account for his actions," said Mr Ross.
A senior Tory source said Mr Johnson looked "battered and crestfallen" as he spoke to his MPs, and that he had "lost what made him so successful with his party".
Backbencher Sir Roger Gale - a frequent critic of Mr Johnson - said that politically the PM was now "a dead man walking".
Minister Rachel Maclean warned there were consequences for those who have broken the law regarding coronavirus restrictions.
Speaking to BBC Two's Politics Live, she said: 'The law of the land applies to everybody…including the prime minister. The people that make the laws are also the subject of the laws and that's why we've got this due process of this inquiry find out exactly what went on, and if any laws were broken there will be consequences."
But other Tory MPs rallied behind the prime minister, with Sir Christopher Chope saying he had "never heard such an abject apology" in his time in parliament, and he believed it was "genuinely sincere".
The Commons fell silent at the start of Prime Minister's Questions, as Mr Johnson admitted he had been at the Downing Street party on 20 May 2020 for about 25 minutes, so that he could "thank groups of staff" for their hard work.
He said: "I believed implicitly that this was a work event."
But he added: "With hindsight I should have sent everyone back inside.
"I should have found some other way to thank them, and I should have recognised that - even if it could have been said technically to fall within the guidance - there would be millions and millions of people who simply would not see it that way."
Later, a Downing Street spokesman said the prime minister did not see the email inviting staff to drinks on 20 May 2020.
Mr Johnson sat stony-faced as opposition MPs called for him to quit as prime minister, or for his own MPs to force him out.
In all, the PM faced eight calls to stand down during the Commons question session. Tory MPs, by and large, used the session to ask questions about constituency projects and coronavirus.
Sir Keir Starmer said: "There we have it. After months of deceit and deception, the pathetic spectacle of a man who has run out of road.
"His defence...that he didn't realise he was at a party is so ridiculous that it's actually offensive to the British public.
"He's finally been forced to admit what everyone knew, that when the whole country was locked down he was hosting boozing parties in Downing Street. Is he now going to do the decent thing and resign?"
Mr Johnson said he understood the "rage" of people who had "made huge sacrifices throughout this pandemic" at the thought "that people in Downing Street were not following those rules".
"I regret the way the event I have described was handled. I bitterly regret it. And wish that we could have done things differently."
I don't think the prime minister's statement makes the issue go away at all.
He tried to strike a different tone today - there was no smirk, no swagger, none of the usual Johnson gags.
His admission may have bought him some time, but he is basically pleading with his party to wait for the inquiry to conclude before they make their mind up.
However, some people will see this as being a non-apology apology.
Mr Johnson urged MPs to wait for the outcome of an inquiry by senior civil servant Sue Gray into alleged Covid law-breaking in Downing Street, which he said "will report as soon as possible".
SNP Westminster leader Ian Blackford said that if Mr Johnson had "no sense of shame", then the Tory backbenchers "must act to remove him".
Liberal Democrat leader Sir Ed Davey also called on the PM to resign.
Later, Tory MP Stephen Hammond told BBC Radio 4's The World at One that Mr Johnson had demonstrated "some transparency" by admitting his mistakes.
But he added: "It clearly was not a work event and that is the problem. and I think it is pretty surprising the prime minister wasn't briefed to that effect."
He said his constituents would still feel "let down" having heard "a bit of an apology" from the PM.
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2022-01-12 16:14:06Z
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