Labour has demanded Boris Johnson come to parliament and answer questions about whether he misled MPs over alleged Christmas lockdown rule-breaking.
The prime minister last week told the Commons there was no party and that no Covid rules were broken in Downing Street – shortly before photos emerged of him hosting a Christmas quiz.
Some Conservatives, including Scottish Tory leader Douglas Ross, have said Mr Johnson’s position would be untenable were he found to have lied about the party at his home address.
But Conservative ministers spent Sunday trying to downplay the latest revelations of Mr Johnson’s festivities in No 10.
Education secretary Nadhim Zahawi took to the airwaves to claim the quiz could not have been a party because there was no alcohol.
When it was pointed our that Mr Johnson was flanked by two colleagues in Christmas garb, the minister told the BBC: “I don’t think there’s a rule against recognising Christmas with tinsel or a hat.”
But Labour leader Keir Starmer, who is a former director of public prosecutions, said it appeared that the prime minister had broken the rules.
Sir Keir, asked on the BBC’s Andrew Marr Show whether the prime minister had been breaking the law, said: “Well, it looks as though he was. He must have known that other groups were in other rooms in his own building.”
But the opposition leader stopped short of calling for Mr Johnson to resign over his participation in the event.
Instead he branded the PM the “worst possible leader at the worst possible time”, arguing that trust in new measures to control the fast-spreading omicron variant could have been undermined by recent rule-breaking allegations.
Mr Zahawi meanwhile argued the prime minister’s case, telling Sky News: “What do we see in that picture? We see a prime minister on a virtual quiz night for 10 to 15 minutes, to thank his staff – who by the way had no choice but to come in every single day.
“Sitting in his office with the two people who are closest working with him, no alcohol on the table, not drinking – on a Zoom call or teams call, virtual call – respecting the lockdown rules.
“Many people would have had similar Zoom quiz nights around the country. They are in his office, they work with him.”
Social events outside household bubbles were banned under government rules at the time, including with colleagues who might otherwise have been working together.
Official guidance stated: “You must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier.”
It comes during ongoing anger about a separate party held at Downing Street, which is now the subject of an investigation by the cabinet secretary.
Angela Rayner, Labour’s deputy leader, said: “Despite repeated denials of parties in No 10, it has now transpired that there were numerous parties, gatherings and the prime minister even took part in a festive quiz.
“It appears that Boris Johnson lied to the country and broke the law. It is increasingly clear that the prime minister presided over a culture of ignoring the rules that he told everyone else to follow. It’s time to fess up.
“Boris Johnson and the Conservatives really believe it’s one rule for them, another for everyone else.”
A No 10 spokeswoman said: “This was a virtual quiz. Downing Street staff were often required to be in the office to work on the pandemic response so those who were in the office for work may have attended virtually from their desks.
“The prime minister briefly took part virtually in a quiz to thank staff for their hard work throughout the year.”
Mr Johnson on Sunday night dodged questions from the press over the affair by pre-recording his statement to the country.
