Tory peer and former Conservative Party chairwoman Sayeeda Warsi is calling on those present at an alleged Christmas party at No 10 during lockdown restrictions last year to resign.
Baroness Warsi, who is from Dewsbury, tweeted: "Every minister, parliamentarian & staffer at the #downingstreetparty must resign NOW.
"No ifs no buts. The rule of law is a fundamental value, the glue that hold us together as a nation.
"Once that is trashed by those in power the very essence of our democracy is at stake."
It comes as Boris Johnson is facing calls to "come clean" about the alleged party, as the Government refused to send a minister to defend its position on television.
Leaked footage from No 10's £2.6 million press briefing room emerged on Monday night, which showed former press secretary Allegra Stratton laughing as she appeared to rehearse answers to questions over a lockdown-busting Christmas party.
Watch the full exchange between Ed Oldfield and Allegra Stratton in the mock televised press briefing
The video, which is reported to be from 22 December last year, refers to a party on "Friday" - which would have been 18 December, the same day The Daily Mirror reported there was a staff party where games were played, food and drinks were served, and revelries went on past midnight.
No 10 initially did not say the reports were inaccurate but said all rules had been followed, before later denying any party had taken place.
But the emergence of the video will put Boris Johnson under increasing pressure as he faces Labour leader Sir Keir Starmer at Prime Minister's Questions.
In the footage obtained by ITV News, Ms Stratton and adviser Ed Oldfield, along with other aides, were filmed joking about a "fictional" Downing Street party.
Mr Oldfield can be heard asking Ms Stratton: "I've just seen reports on Twitter that there was a Downing Street Christmas party on Friday night, do you recognise those reports?"
Ms Stratton replied "I went home" before appearing to consider what the correct answer should be.
During the rehearsal, filmed as part of a subsequently-abandoned plan for Ms Stratton to lead televised press briefings, one aide is heard saying: "It wasn't a party, it was cheese and wine."
"Is cheese and wine all right? It was a business meeting," Ms Stratton replied, to laughter in the room.
Ms Stratton then noted "this is recorded", adding: "This fictional party was a business meeting ... and it was not socially distanced."
The Metropolitan Police confirmed officers are reviewing the leaked video in relation to "alleged breaches" of coronavirus regulations.
In response to ITV's report, a Downing Street spokesman said: "There was no Christmas party. Covid rules have been followed at all times."
The challenge for Downing Street was laid bare by no Cabinet minister being offered to represent the Government in morning broadcast interviews, and there were questions over whether a suggested press conference to mark one year since the first coronavirus vaccine was delivered would go ahead.
As well as Health Secretary Sajid Javid pulling out of national interviews, vaccines minister Maggie Throup is understood to have pulled out of a planned round of regional television interviews.
Former Health Secretary Matt Hancock told ITV's Good Morning Britain: "What I know is that the Prime Minister said that no rules were broken. And nobody's suggesting that he was at this party."
But a former vice-chairman of the 1922 Committee of backbench Tory MPs has said the Government is likely to now find it "almost impossible" to introduce "very proscriptive" Covid-19 restrictions due to the saga.
Sir Charles Walker, MP for Broxbourne in Hertfordshire, told Times Radio: "I think now that, going forward, any measures will be advisory. I think it would be very difficult to enshrine them in law and then once again ask our poor police forces to enforce them."
He added: "To be very proscriptive about this now, particularly as we've had such a successful vaccine rollout... is much more difficult, and was always going to be much more difficult. And the events of the last 24 hours make it probably almost impossible now."
While chairman of the Commons Education Committee and Tory MP for Harlow Robert Halfon said: "I certainly think that those who were doing the video should apologise for the insensitivity of it when people were suffering and struggling all through that time."
But he stopped short of calling for the Prime Minister himself to apologise.
Ministers are yet to explain how the alleged bash complied with the rules in place at the time, despite coming under pressure since an initial report in the Daily Mirror.
The newspaper said two events took place in No 10 in the run-up to the festive season last year, including Mr Johnson giving a speech at a leaving do during November's lockdown.
The other was said to be the staff party in December.
At the time, the Tier 3 rules explicitly banned work Christmas lunches and parties where it is "a primarily social activity and is not otherwise permitted".