Boris Johnson stands accused of personally attending a drinks party in the garden of No 10 during the first lockdown.
Downing Street did not immediately deny a report in the Sunday Times alleging that the Prime Minister attended the potentially rule-breaking event with wife Carrie in May 2020.
The newspaper cited three sources stating Mr Johnson’s principal private secretary, Martin Reynolds, emailed officials with an invite adding “BYOB”, meaning bring your own bottle, or booze.
Senior civil servant Sue Gray is investigating the May 20 event as part of her inquiry into numerous allegations of rule-breaking events being held in No 10 during the coronavirus pandemic.
Allegations of that gathering, said to have been attended by 40 people, emerged this week when Dominic Cummings said he had warned at the time the “socially distanced drinks” would likely be against the rules and “should not happen”.
But the front page report suggesting Mr Johnson himself was present will add to the seriousness of the claims.
The Prime Minister imposed England’s first lockdown to combat Covid-19 in March 2020 and it was not until June 1 that groups of up to six people were allowed to meet outdoors.
Deputy Labour leader Angela Rayner said: “The culture of total disregard for the rules seems to have been embedding into life in Downing Street from the very start of the pandemic.
“When much of the country was struggling with empty shelves and a total lockdown with no meeting with other allowed, it seems No 10 we’re hosting parties from the very start.
“Boris Johnson and his team are taking the country for fools. We need the report from Sue Gray’s independent investigation as soon as possible and Boris Johnson needs to face the consequences of his actions.”
Ms Gray, a senior Cabinet Office official, also expanded her investigation to the May 15 2020 garden gathering revealed by a leaked photo showing the Prime Minister and staff sat around cheese and wine.
Mr Cummings, who was present in the picture, insisted in a blog post on Friday that there was nothing “illegal or unethical” about that day, which he insisted was a staff meeting.
But the Vote Leave veteran, who departed as the chief adviser in Downing Street in November 2020, wrote: “On Wednesday 20 May, the week after this photo, a senior No 10 official invited people to ‘socially distanced drinks’ in the garden.”
Mr Cummings said he and at least one other special adviser “said that this seemed to be against the rules and should not happen”, adding that he issued the warning “in writing so Sue Gray can dig up the original email”.
“We were ignored. I was ill and went home to bed early that afternoon but am told this event definitely happened,” he continued.
No 10 and a spokeswoman for Mrs Johnson have both been asked to comment on the latest claim.