Boris Johnson looks to have broken the law when he hosted a Christmas quiz at Downing Street during lockdown, Labour's leader has said.
Keir Starmer, a former director of public prosecutions, said it "looks as though" the prime minister was in breach of the rules.
Pictures published by the Sunday Mirror show the PM flanked by colleagues and draped in tinsel in No.10, with others apparently taking part online.
Downing Street has confirmed that the PM "briefly" took part in the quiz, though they stressed the virtual nature of some participants.
Defending the PM on Sunday morning television Nadhim Zahawi, who was at the time the vaccines minister, stressed that no alcohol had been solved at the event.
Asked about the apparent social nature of the meeting, he told the BBC: “I don’t think there’s a rule against recognising Christmas with tinsel or a hat.”
But Sir Keir, asked on the BBC’s The 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.
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.”
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.”
