Downing Street has denied a Christmas party was held and Johnson has insisted no rules were broken.
British Prime Minister Boris Johnson said he was "furious" and had ordered an inquiry after a leaked video showed senior staff members joking about holding a Christmas party in breach of UK lockdown rules.
It comes after days of Downing Street attempts to play down the story – which was shown on a UK television network - which suggested his staff had flouted coronavirus rules.
"I understand and share the anger up and down the country at staff seeming to make light of lockdown rules," Johnson told MPs.
"I was also furious to see that clip. I apologise unreservedly for the offense that it has caused up and down the country and I apologise for the impression it gives."
British media outlets claim that the party took place on December 18, when restrictions in London prohibited most indoor gatherings, and a day before Johnson tightened the rules so they ruled out Christmas celebrations for millions of people.
Johnson said he had ordered Britain's top civil servant, Simon Case, to investigate, and said anyone found to have broken the rules would be disciplined.
He added that he had been "repeatedly assured ... that there was no party and no COVID rules were broken."
Meanwhile, Britain's opposition Labour Party wrote to London's Metropolitan Police asking them to investigate reports that two parties were held at No. 10 Downing Street in the run-up to last Christmas.
The police force said that it was aware of "widespread reporting" of "alleged breaches of health protection regulations" at a government building last November and December.
It said although it did not routinely investigate "retrospective" breaches of COVID-19 regulations, it was considering the complaints.
'The party is over'
On Wednesday, the Westminster leader of the Scottish National Party called for Johnson to resign.
"It really isn't acceptable," Ian Blackford, said in response to the video. "Here we have No. 10, a government in London, breaching its own COVID rules and then joking about it on a video."
Blackford called for an investigation into the allegations and for the police to look into the alleged breach of coronavirus rules.
"On the basis of this behaviour, because it's not just about this, it fits a pattern of a prime minister who repeatedly lies, a prime minister that can't be trusted, he should go and he should go now," he added.
UK Labour Party leader Sir Keir Starmer also called on Johnson to "step up" and apologise. "Unfortunately, we've got a prime minister who is socially distanced from the truth," Starmer said.
Downing Street has denied a Christmas Party was held and Johnson has insisted no rules were broken.
On Wednesday, PM Johnson's former spokesperson, Allegra Stratton, announced her resignation following the publication of the video and has apologised
"My remarks seemed to make light of the rules, rules that people were doing everything to obey," she said.
"The British people have made huge sacrifices in the battle against COVID-19, I fear now that my words have become a distraction from that fight," she said in a televised statement outside her home.
Political commentator Jonathan Lis told Euronews that Stratton had been "thrown under the bus".
"This isn't just the Prime Minister sanctioning parties, or knowing about parties, it's about [reportedly] him attending parties and then trying to cover it up and lying about it."
"I don't think the announcing of an investigation, feigning shock, is going to work," Lis said.
Watch the full interview with Jonathan Lis in the video player above.