British Prime Minister Boris Johnson was under fire Wednesday after a video emerged of his senior aides joking about holding a Christmas party at Downing Street last year when social gatherings were banned under Covid rules.

The government has been accused repeatedly of hypocrisy over breaches of lockdown rules and in a major scandal health secretary Matt Hancock resigned in June after revelations that he broke coronavirus restrictions during an affair with an aide.

The video, obtained by broadcaster ITV News, shows Johnson's then press secretary Allegra Stratton, advisor Ed Oldfied and other staff joking about "a fictional party" during a rehearsal press conference on December 22, with no media present.

In the leaked footage, Stratton is seen answering questions about a Downing Street Christmas party the previous Friday -- when the alleged rule-breaking gathering took place.

"This fictional party was a business meeting and it was not socially distanced," she laughs over joking exchanges about "cheese and wine".

At that time, London was under strict Covid restrictions and indoor social gatherings of two or more people were banned.

In response to the video, Downing Street insisted: "There was no Christmas party. Covid rules have been followed at all times."

Keir Starmer, leader of the opposition Labour Party, blasted the government for its "shameful" actions at a time when Britain was under lockdown.

"People across the country followed the rules, even when that meant being separated from loved ones. They had a right to expect the government was doing the same," Starmer tweeted, with a link to the video.

"To lie and to laugh about those lies is shameful. We have a Prime Minister who's socially distanced from the truth."

'Upsetting and shameful'

The leader of the Scottish National Party, Ian Blackford, said the prime minister could not be trusted and called for him to resign.

"Here we have Number 10, a government in London, breaching its own Covid rules and then joking about it on a video," Blackford told STV News.

"It really isn't acceptable and I have to say, unfortunately, that on the basis of this behaviour ... he should go, and he should go now."

Some lawmakers from Johnson's Conservative Party also wanted answers.

"The No.10 party has all the hallmarks of another 'Barnard Castle' moment," North Thanet MP Roger Gale tweeted, referring to an incident last year when Johnson's then chief aide Dominic Cummings drove hundreds of kilometres during a lockdown, triggering outrage over perceived hypocrisy by the government.

"No.10 clearly has some serious questions to answer. Fast."

During the mock press conference, Oldfield mentions reports about a Downing Street party and asks: "Would the prime minister condone having a Christmas party?" to which Stratton says "What's the answer?"

Oldfield says he doesn't know as a Downing Street staffer says to laughter that "It wasn't a party... it was cheese and wine."

Covid-19 Bereaved Families for Justice said it was "the behaviour of people who think they're above us."

"There are simply no words to describe how upsetting and shameful it is to then hear Boris Johnson's team laughing about breaking the rules they had made, whilst others followed them and could only say goodbye to their loved ones through a screen," it said in a statement.