Allegra Stratton, who was most recently Johnson's COP26 spokeswoman, was his press secretary at the time the video was recorded in December 2020.

In the video aired by ITV, Stratton was shown at a rehearsal for a daily briefing laughing and joking about a reported gathering at a time when tens of millions of people across the UK were banned from meeting family and friends to celebrate Christmas, or even from bidding farewell to dying relatives.

"My remarks seemed to make light of the rules, rules that people were doing everything to obey. That was never my intention. I will regret those remarks for the rest of my days," a tearful Stratton told reporters outside her home.

"I understand the anger and frustration that people feel. To all of you who lost loved ones, who endured intolerable loneliness and who struggled with your businesses - I am truly sorry and this afternoon I am offering my resignation to the prime minister."

Johnson earlier on Wednesday apologised after the video of his staff surfaced.

Johnson said he had been furious to see the clip and that, since allegations emerged in the media, he had been repeatedly assured that there had not been a party.

"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, adding that there would be disciplinary action if it was found rules were broken.

Johnson and his ministers have repeatedly denied any rules were broken by the gatherings in late 2020 although the Mirror newspaper said Johnson spoke at a leaving party and that his team had a wine-fuelled gathering of about 40 to 50 people.

But in a video aired by ITV, Stratton was shown at a 2020 Downing Street rehearsal for a daily briefing laughing about the 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 UK flags at an official Downing Street lectern, chuckles and says: "I went home."

She then laughs and smiles.

"Hold on. Hold on. Um, er, arh," she says, appearing lost for words and looking up.

Reaction to the video was sharp with many people on Twitter expressing disgust that Downing Street appeared to be laughing about breaking rules.

Some questioned whether or not the public should obey Johnson if he imposes more COVID-19 restrictions.

Nearly 146,000 people have died from COVID-19 in the United Kingdom and Johnson is weighing up whether to toughen curbs after the discovery of the new Omicron coronavirus variant.

Johnson has faced intense criticism in recent months over his handling of a sleaze scandal, the awarding of lucrative COVID-19 contracts, the refurbishment of his Downing Street flat and a claim he sought to ensure pets were evacuated from Afghanistan during the chaotic withdrawal of foreign forces.

Opposition Labour Party leader Keir Starmer said the video was an insult to those who had followed lockdown rules when it meant being separated from their families over Christmas.

"They had a right to expect that the government was doing the same. To lie and to laugh about those lies is shameful," Starmer said in a statement.

"The prime minister now needs to come clean and apologise."

Ian Blackford of the Scottish National Party, the second-biggest opposition party in parliament, called for Johnson to step down.

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

Australian Associated Press