The Labour leader said it was ‘very hard’ to see how the virtual quiz was ‘compliant with the rules’ at the time.
Sir Keir Starmer, a former director of public prosecutions, said it was “very hard” to see how the virtual quiz was “compliant with the rules”.
The comments come after the Sunday Mirror published a picture of Boris Johnson reportedly playing quiz master while flanked by colleagues, one of whom is draped in tinsel, in the No 10 library while London was in Tier 2 restrictions.
Downing Street has confirmed that the Conservative Party leader had “briefly” taken part “virtually” in the quiz.
Tier 2 restrictions stated that there could be no mixing of households indoors, apart from support bubbles, and a maximum of six people outside.
The quiz is said to have taken place on December 15 – only three days before an alleged rule-breaking Christmas party in No 10, which is currently being investigated by the Cabinet Secretary, Simon Case
Official guidance at the time of the quiz set out that there should not be work lunches or parties “where that is a primarily social activity” and was not exempted for “work purposes”.
Sir Keir, asked on the BBC’s The Andrew Marr Show whether the Prime Minister had been breaking the law at the time the picture was taken, said: “Well, it looks as though he was.
“He must have known that other groups were in other rooms in his own building.”
The Sunday Mirror quoted a source who claimed many staff were huddled by computers in their Downing Street offices, conferring on questions and drinking alcohol while the quiz was taking place.
It reported Mr Johnson surprised staff by turning up on screen as quiz master for one round lasting between 10 and 15 minutes.
Opposition leader Sir Keir stopped short of calling for Mr Johnson to resign over his participation in the event.
He, instead, called him 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.
Cabinet minister Nadhim Zahawi said Labour had adopted the “wrong” stance on the quiz as he defended the Prime Minister against suggestions of having flouted the regulations.
The Education Secretary told Sky News’ Trevor Phillips On Sunday programme: “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 are the two people who are closest working with him, no alcohol on the table, not drinking, on a Zoom call or a Teams call, a virtual call respecting the lockdown rules.
“Many people would have had similar Zoom quiz nights around the country.”
He added that there was not “a rule against recognising Christmas with tinsel or a hat”, as the staff pictured on either side of Mr Johnson appeared to be doing.
Mr Zahawi said there had been “hype” around Government party claims, with two alleged Downing Street events currently being probed, but that the picture would allow the public to “make their mind up” for themselves.
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.”
The quiz reports come as Mr Case, who is also head of the Civil Service, investigates three alleged rule-breaking Government gatherings last winter.
Staff reportedly held a Christmas bash in No 10 on December 18, with a leaked video – filmed four days after the alleged drinks – showing senior Downing Street aides joking about a “fictional” party.
A second reported Downing Street event – an aide’s leaving do, which is said to have taken place on November 27 – was allegedly attended by the Prime Minister, whom the Mirror said made a speech.
Mr Case is investigating both alleged events, along with a festive celebration arranged at the Department for Education, which officials have expressed regret over.
The Cabinet Office said Mr Case has the power to widen the investigation further if evidence of more potential events comes to light.