Police have a photograph of Boris Johnson holding a can of a beer at his lockdown birthday party in June 2020, the Mirror has been told.
The Prime Minister is pictured standing next to Chancellor Rishi Sunak, who is holding a soft drink, in No10’s Cabinet Room.
Sources said Mr Johnson appeared to be raising his can of Estrella beer towards the camera in a toast.
The bombshell image was thought to have been taken by Mr Johnson’s official taxpayer-funded photographer who was documenting the event.
It is thought to be among 300 pictures submitted to the Metropolitan Police as evidence of rule-busting gatherings by senior civil servant Sue Gray.
These are believed to include photos taken at parties and those taken from security-system cameras showing when people entered and exited buildings.
Multiple images taken by the official No 10 photographer, Andrew Parsons, are also believed to have been handed over to Scotland Yard.
Downing Street has admitted staff “gathered briefly” at a surprise birthday celebration organised by Carrie Johnson - but said the PM only stayed 10 minutes.
But at the time social gatherings indoors were still forbidden under lockdown laws as the country emerged from the first wave of the pandemic.
Barrister Adam Wagner, an expert in Covid laws, said: “The legal question for the Prime Minister is whether he participated in the gathering.
“The fact that he was photographed holding a beer strongly suggest he did and therefore committed a criminal offence of the regulations”.
The picture shows the two senior politicians surrounded by No 10 aides in the Cabinet Room but no birthday cake, sources said.
Mrs Johnson led up top 30 staff in a chorus of happy birthday before tucking into picnic food from M&S, it has been claimed.
Lulu Lytle, the interior designer behind lavish renovations of the Downing Street flat, also briefly attended.
The Chancellor is understood to have attended the event as he turned up for a Covid meeting, but had not been invited in advance.
Mr Sunak, hotly tipped as next PM, admitted this week that the ‘Partygate’ row had damaged trust in the Government.
Initial reports that Mr Johnson was “ambushed” with a birthday cake were later denied by his close allies.
Later that evening, the PM and his wife were said to have hosted family friends in their residence upstairs in an apparent further breach of the rules.
Downing Street denied these claims - saying the PM hosted a small number of family members outside instead.
Mr Parsons, one of three official photographers funded by the taxpayer, is a regular presence in No 10, taking snaps of the PM at some of his most significant moments.
These included the dramatic picture of Mr Johnson roaring when the exit poll for his historic 2019 election victory was revealed.
Labour ’s deputy leader Angela Rayner said: “The Downing Street photographer is funded by the taxpayer, and the public have every right to see the photos they pay to have taken, including any of the Prime Minister’s birthday party.
“Following the Met Police investigation, the Sue Gray report must be published in its entirety with all accompanying evidence - photos, videos and testimony.”
A No 10 spokesman admitted: “A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday.
“He was there for less than ten minutes.”
