British Prime Minister Boris Johnson apologised and his adviser resigned on Wednesday after a video emerged of senior aides joking about a Christmas party at Downing Street last year when social events were banned under Covid-19 rules.

The leaked footage appears to contradict more than a week of denials by Johnson and his ministers that a party took place, following newspaper reports that dozens of staff had attended an evening-long gathering on December 18, 2020.

The embattled leader, facing calls for his resignation from some opposition politicians during a raucous parliamentary questions session, vowed "disciplinary action" would be taken if the investigation found rules had been broken.

"I understand and share the anger up and down the country at seeing No 10 staff seeming to make light of lockdown measures," he said.

"I apologise unreservedly for the offence that it has caused... and I apologise for the impression that it gives."

Johnson added he had been "repeatedly assured" that there was no party inside his Number 10 Downing Street office and that no rules were broken.

https://www.youtube.com/watch?v=4ipEB80UxII

Johnson's adviser Allegra Stratton, a former political journalist who was serving as his press secretary at the time the footage was recorded, swiftly tendered her resignation.

The footage showed her referring jokingly to a "fictional party" while preparing for a press conference. 

In a tearful statement, Stratton acknowledged her comments "seemed to make light of the rules" and said she would "regret those remarks for the rest of my days". 

"I understand the anger and frustration that people feel", she said, while not specifying whether a party took place.

- Police will not probe -

Labour party leader Keir Starmer told the prime minister that his account strained credulity.

"They knew there was a party, they knew it was against the rules, they knew they couldn't admit it, and they thought it was funny," he told MPs.

"Does the prime minister think he has the moral authority to lead and to ask the British people to stick to the rules?" he asked.

The country's most senior civil servant, Cabinet Secretary Simon Case, will lead the newly announced probe.

Case "has been asked to establish the facts on any events on (December) 18, and that's what he will start work on," Johnson's spokesman told reporters. The team will "rightly" set its timeframe, "but obviously we want it (a report) as soon as possible."

London's Metropolitan Police said in a statement Wednesday they would not be opening an investigation as the footage "does not provide evidence of a breach of the health protection regulations," while also citing "our policy not to investigate retrospective breaches".

The alleged party is the latest instance of apparent government hypocrisy over restrictions after several previous scandals, including health secretary Matt Hancock resigning in June for breaking coronavirus curbs during an affair with an aide.

The incendiary footage shows Stratton fielding questions during a rehearsal press conference on December 22 with no media present.

The group laugh and trade jokes over a "fictional party" four days earlier, which reportedly involved food, drink, games and "secret Santa" gift-giving.

"This fictional party was a business meeting and it was not socially distanced," Stratton laughs over joking exchanges about "cheese and wine" and whether the prime minister would "condone" such an event.

At the time, London was under strict restrictions and indoor social gatherings of two or more people were banned, while office parties were specifically outlawed.

- 'Isn't a laughing matter' - 

The video, which was leaked to broadcaster ITV News late Tuesday, has led main news bulletins, drawn millions of views online and sparked condemnation from across the political spectrum.

Conservative lawmakers were among those demanding answers.

"This isn't a laughing matter," Tory MP Roger Gale told the BBC.

The scandal has echoes of an infamous incident earlier in the pandemic involving Johnson's then-chief aide Dominic Cummings, who drove hundreds of kilometres to stay away from his London home during a lockdown.

With the Omicron variant spreading fast in Britain, some Conservative lawmakers said the latest scandal would make reintroducing curbs "much more difficult".

"The events of the last 24 hours make it probably almost impossible now," influential backbench MP Charles Walker told Times Radio.

But Johnson later Wednesday announced new virus measures for England including advice to work from home, the introduction of mandatory Covid passes for crowded areas, and mask-wearing inside most public indoor venues.