Boris Johnson has been confronted over a new picture revealing an open bottle of what appears to be champagne or sparkling wine at a No 10 “virtual” Christmas quiz during lockdown.

The prime minister is flanked by three members of staff, one wearing tinsel and another a Santa hat, at the event on December 15, 2020, the image shows.

In the Commons, Mr Johnson was told it looked like “one of the Christmas parties” that he had denied took place – but insisted the claim was “completely in error”.

The gathering is not one of the 12 being investigated by the Metropolitan police, a probe which has left the prime minister’s survival in office hanging by a thread.

In the Commons, Mr Johnson was urged to “refer the matter to the police”, but replied: “That’s precisely the point – it already has been submitted for investigation”.

But Adam Wagner, the barrister and expert in lockdown restrictions, said: “We can now see the food and alcohol and an extra person. I think there is no longer any justification for the police not to investigate this event.”

At the time, then-vaccines minister Nadhim Zahawi argued the gathering could not have been a party – because no alcohol was present.

The publication of the photo – by the Daily Mirror – will add to Tory MPs’ fears of a drip-drip of more damaging evidence of Covid rule-breaking in No 10, as the Met inquiry continues.

Dominic Cummings, the ousted Johnson aide, who has claimed more evidence will emerge, tweeted: “There’s waaaaay better pics than that floating around, incl in the flat.”

That is a reference to the separate alleged party hosted by the prime minister’s wife, in November 2020, when ABBA songs were heard being played, at loud volume.

At a stormy prime minister’s questions, Mr Johnson was also warned, by former Tory chief whip Mark Harper, not to stall on publishing the full Sue Gray report – once the Met inquiry is completed.

“I will immediately publish in full whatever Sue Gray gives me,” MPs were told – although it is not clear whether that report will include the 300-plus photos and 500-plus pages of written evidence she has gathered.

Mr Johnson was accused of personally breaking Covid laws by hosting the festive event in December 2020 – but No 10 argued quiz participants had joined virtually.

At the time, London was under Tier 2 regulations banning any social mixing between two or more people from different households.

Official guidance also 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.”

On the desk in front of Mr Johnson – from where he read out quiz questions to teams in different rooms – is what appears to be champagne and a half-eaten packet of crisps.

Two days after the Mirror broke the story, the prime minister said: “I can tell you once again that I certainly broke no rules.”