Boris Johnson has apologised over a party held at Downing Street during the first coronavirus lockdown, as he faced MPs at Prime Minister's Questions.
In a heated session in parliament on Wednesday, opposition Labour leader Sir Keir Starmer called on Johnson to resign.
Some Tory MPs, including Scottish Conservative leader Douglas Ross, have also agreed that the prime minister needs to step down.
It comes amid public and political fury over the latest allegations that the prime minister and his staff flouted lockdown restrictions by having a party in May 2020.
Watch PMQs back as it happened in the video player above.
It's claimed that dozens of Johnson's staff, as well as the prime minister himself along with his wife, attended the event in the Downing Street garden during the first coronavirus lockdown, at a time when social mixing was banned.
"I want to apologise," Boris Johnson said as he appeared before MPs. "I know that millions of people across this country have made extraordinary sacrifices over the last 18 months," adding that he recognised their "anguish" at being unable to mourn relatives and live their lives normally.
"I know the rage they feel with me and with 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."
"There were things that we simply did not get right, and I must take responsibility," the prime minister added. Admitting that he attended the drinks party, he said he was there for 25 minutes and "thought it was a work event".
Starmer described Johnson's response as "offensive to the British public", who were obeying lockdown rules while the prime minister and his staff were "hosting boozy parties in Downing Street". He accused Johnson of "lying through his teeth" and called on him to "do the decent thing, and resign".
The prime minister accused the opposition leader of making a political point, saying he would not pre-empt the outcome of the current internal inquiry. A senior civil servant is investigating reports that Johnson's staff held several social events during lockdowns.
The leader of the Scottish National Party (SNP) in parliament, Ian Blackford, accused Boris Johnson of "betraying the nation's trust" and "treating the public with contempt".
As well as opposition figures, even some within Johnson's own ruling Conservative Party have called on him to quit — and his answers will be closely scrutinised.
According to ITV News, Johnson's principal private secretary sent an email to around 100 people, inviting them to "bring your own booze" to "socially distanced drinks". Some 30-40 people are said to have attended.
A social media post by the Metropolitan Police on the same day of the party reminded people that they could enjoy being outside as long as they were "on your own / with people you live with / just you and one other person".
Millions of people around the country desisted from holding social events, during the early stages of the the pandemic, months before the vaccination rollout. Thousands of people died, while many were unable to visit sick relatives or attend funerals.
Government ministers and some Tory politicians have reserved judgement on the prime minister in media interviews, citing the ongoing internal inquiry into alleged parties at Downing Street during lockdowns.
But many in private are furious, and after a series of scandals last year, some have publicly questioned whether Johnson should remain as party leader and prime minister.
Johnson has previously repeatedly denied that lockdown guidelines were broken by his staff, including when he has appeared before parliament.