Boris Johnson is accused of personally attending a drinks party in the No 10 Garden during the first lockdown in 2020.

Downing Street did not immediately deny a report that featured in the Sunday Times. It alleges the Prime Minister attended the potentially rule-breaking event with his wife Carrie in May 2020.

The Sunday Times cited three sources stating 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 pandemic.

Allegations claim the gathering was attended by 40 people.

Allegations emerged this week when Dominic Cummings said he had warned at the time that “socially distanced drinks” would likely be against the rules and “should not happen”.

The report suggests Johnson himself was present which adds to the seriousness of the claims.

The first lockdown was imposed by the Prime Minister in March 2020. It was not until June 1 that groups of up to six people were allowed to meet outdoors while socially distanced.

Bury Times: Boris Johnson faces backlash after alleged gathering in No 10 garden (PA)Boris Johnson faces backlash after alleged gathering in No 10 garden (PA)

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, has also expanded her investigation to the May 15, 2020, No 10 Garden gathering revealed by a leaked photo showing the Prime Minister and staff sitting 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 Cummings, 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.”

He claimed 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.