Johnson admitted for the first time that he had attended the party at 10 Downing Street in May 2020 when COVID-19 rules limited social gatherings to a bare minimum, saying he understood public anger.
"I know the rage they feel with me over the government I lead when they think that in Downing Street itself the rules are not being properly followed by the people who make the rules," an ashen-faced Johnson told parliament.
"I offer my heartfelt apologies," he said.
Johnson said he regretted his action and had thought the gathering was a work event - drawing jeers and laughter from opposition lawmakers.
"I went into that garden just after six on the 20th of May 2020 to thank groups of staff before going back into my office 25 minutes later to continue working," Johnson said.
"With hindsight, I should have sent everyone back inside."
Leaders of all the main opposition parties called for his resignation, while the Conservatives' leader in Scotland became the first figure in his party to say Johnson should now quit.
Labour Party leader Keir Starmer said the public - who handed Johnson a landslide election victory in December 2019 after he promised to secure Britain's exit from the European Union - thought him a liar.
"The party's over, prime minister," Starmer told him.
"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 is actually offensive to the British public."
'NOT SUNK YET'
Anger has grown since ITV News reported Johnson and partner Carrie mingled with about 40 staff in the Downing Street garden after his Principal Private Secretary Martin Reynolds sent an invitation asking attendees to "bring your own booze". Johnson's press secretary said the prime minister had not seen that email.
Numerous people, including some lawmakers, have described how the rules kept them from the bedsides of dying loved ones last May in contrast to the events in Downing Street.
Some of Johnson's own Conservative lawmakers had said his response on Wednesday to the growing furore would determine his future.
"He's taken on a lot of water and is listing but not quite sunk yet," one of them told Reuters.
Senior ministers rallied round Johnson to express support on social media, but other lawmakers were unconvinced, most notably Scottish Conservative leader Douglas Ross.
"Regretfully, I have to say his position is no longer tenable," Ross told Sky News, having earlier spoken to Johnson. Sky said he would submit a letter of no confidence in the prime minister.
To trigger a leadership challenge, 54 of the 360 Conservative MPs in parliament must write letters of no confidence to the chairman of the party's "1922 Committee".
"It sounds to me, I'm afraid, very much as though politically the prime minister is a dead man walking," said Roger Gale, another Conservative lawmaker who has also written a letter calling for Johnson to face a challenge.
Just two years ago, Johnson was riding high: he had secured the biggest Conservative majority since Margaret Thatcher's in 1987 after promising to get Brexit done. He had led the campaign to leave the EU in 2016's referendum.
But a series of missteps over everything from sleaze scandals and the opulent refurbishment of his flat to his handling of COVID-19 and now Downing Street parties have drained his political capital.
Two snap opinion polls on Tuesday showed well over half of respondents thought Johnson should resign. Last month, the Conservatives lost a parliamentary seat they had held for almost 200 years while the party's comfortable lead over Labour in opinion polls has evaporated.
Bookmakers slashed their odds on Johnson being replaced as prime minister this year, with local elections in May viewed as another moment of jeopardy.
When details of the gathering first emerged, Johnson said he could not comment until a senior official, Sue Gray, concludes an investigation into other allegations - initially denied - that he and his officials held rule-breaking parties.
In response to the calls for his resignation, he again deferred to Gray's investigation.
"I cannot anticipate the conclusions of the current inquiry, I have learned enough to know that there were things we simply did not get right. And I must take responsibility," he said.
Opponents said he had not apologised for the party itself which Johnson said on Wednesday "could be said technically to fall within the guidance", but was simply sorry he had been found out.
While parliament resounded with demands for his head, Johnson's biographer Andrew Gimson said he was unlikely to quit unless forced out by his parliamentary colleagues.
"He will be looking for a way through this. He is not the resigning type," Gimson said.