Police have reportedly been handed a photo of Boris Johnson holding a beer at his lockdown birthday party.
The PM was pictured raising a can of Estrella towards the camera at the June 2020 event in the No 10 Cabinet Room, according to The Daily Mirror.
He is said to be standing next to Rishi Sunak in the image taken by an official Downing Street photographer.
Rules at the time banned most indoor gatherings with more than two people.
The photo is said to have been among 300 the Metropolitan Police is examining as part of its investigation into Downing Street parties.
The Met is looking into 12 gatherings and whether they broke Covid restrictions.
The photograph was reportedly taken by Mr Johnson’s taxpayer-funded official photographer Andrew Parsons.
Mr Sunak, who has previously said he was in the room for a Covid meeting, is understood to be holding a soft drink in the image.
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”.
Downing Street has already admitted a birthday celebration took place during the first lockdown.
No 10 said staff “gathered briefly” in the Cabinet Room following a meeting, and the PM was there “for less than 10 minutes.”
But ITV News reported the prime minister’s wife, Carrie Johnson had organised the surprise get-together complete with a chorus of “happy birthday” on the afternoon of 19 June 2020.
It was alleged 30 people attended and shared cake despite social mixing indoors being banned.
Interior designer Lulu Lytle admitted attending but insisted she was only present “briefly” while waiting to talk to Mr Johnson about the lavish refurbishments she was carrying out to the couple’s flat above No 11.
ITV News also reported later that evening family friends were hosted upstairs to continue the PM’s 56th birthday celebrations in his official residence.
No 10 denied this, saying: “In line with the rules at the time the Prime Minister hosted a small number of family members outside that evening.”
Asked about the reports of the PM’s birthday photo, No 10 said it could not comment while the Met Police’s investigation was ongoing. The Treasury was also contacted for comment.