For more than a week, Johnson and his team have repeated that no rules were broken in late 2020 after the Mirror newspaper reported there had been several parties including a wine-fuelled gathering of 40 to 50 people to mark Christmas.

On Wednesday, he said he was furious over the video, which was shown by ITV late on Tuesday, but that he had been repeatedly assured there had not been a party.

Opposition Labour Party leader Keir Starmer accused Johnson of "taking the public for fools", while Ian Blackford of the Scottish National Party called for Johnson to resign. It is the latest misstep by an administration which has been criticised over its handling of a sleaze scandal, the awarding of COVID contracts, the refurbishment of Johnson's Downing Street flat and the chaotic evacuation from Afghanistan.

With reports that the government could implement tougher COVID-19 measures as early as Thursday to try to slow the spread of the Omicron coronavirus variant of the coronavirus, it could also persuade many people to ignore any new rules. "I apologise unreservedly for the offence that it has caused up and down the country and I apologise for the impression that it gives," he told parliament.

Disciplinary action would be taken if it was found that rules were broken, he said.

"But I repeat ... that I have been repeatedly assured since these allegations emerged, that there was no party and that no COVID rules were broken." He also pledged to "get on with the job", accusing the opposition for trying to "muddy the waters about events or non-events of a year ago". After days of denials, the video aired by ITV, Allegra Stratton, who was then Johnson's press secretary, was shown at a 2020 Downing Street rehearsal for a daily briefing laughing and joking about a reported gathering.

In the video, a Johnson adviser asks Stratton: "I've just seen reports on Twitter that there was a Downing Street Christmas party on Friday night — do you recognise those reports?"

Stratton, standing before British flags at an official Downing Street lectern, says: "I went home." She then laughs and smiles. "Hold on. Hold on. Um. Er. Arh." She appears lost for words and looks up.

At the time of the Downing Street gathering, tens of millions of people across Britain were banned from meeting close family and friends for a traditional Christmas celebration - or even from bidding farewell to dying relatives.

Nearly 146,000 people have died from COVID in the United Kingdom and Johnson is weighing up whether to toughen curbs after the discovery of the new Omicron coronavirus variant.

Opposition leader Starmer said it was obvious what had happened at Downing Street. "Ant and Dec are ahead of the prime minister on this," Starmer said - referring to a popular British comedy duo who have made jokes about the outcry - to loud laughter from opposition lawmakers in parliament.

"The prime minister has been caught red-handed," Starmer said in an exchange with Johnson on the floor of the House of Commons.

Conservative Party lawmaker Roger Gale said that if the House of Commons had been deliberately misled over the party then it would be a resignation matter.

But another Conservative lawmaker said that while the mood in the ruling party was poor, there was not the strength of feeling yet for a move against Johnson.

Johnson is also facing questions about whether he sought to ensure that pets were evacuated from Kabul during the Western pullout in August while Afghan people trying to seek refuge abroad were left behind.