On Monday Adam Wagner, a leading lawyer and expert on Covid regulations, said: “This looks unlikely to be legal for attendees. Being outside the home was illegal at the time unless (the only potentially relevant exception) it was for the need to work. ‘Socially distanced drinks’/’BYOB’ don’t sound like work.”

Angela Rayner, Labour’s deputy leader, said: “It is terrible and I think many people that see the evidence now will not only think that Boris Johnson’s lies are catching up with him, but will see it as absolutely despicable that when they were actually told to follow the rules, Boris Johnson and No 10 were breaking the rules.

“It is disgraceful and he should be ashamed.”

On Monday, Liberal Democrat spokesman said the party planned to report the leaked email to the Metropolitan Police.

The force has previously said it does not typically investigate historic allegations that lockdown rules have been breached.

On Monday night, a Met spokesman said: “The Metropolitan Police Service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on 20 May 2020 and is in contact with the Cabinet Office.”

It is understood that the email will now form part of the official probe being carried out by Ms Gray.

In recent days, her team has been asking a number of officials in No 10 about the use of the garden and the consumption of alcohol.

On Friday, she officially expanded her investigation to include the two gatherings in the garden after Dominic Cummings, the former No 10 senior adviser, said he had told Mr Reynolds that he believed the event breached lockdown rules and should not go ahead.

Despite raising the issue, Mr Cummings said he was “ignored”.

Pressure on Martin Reynolds

Government sources said it appeared inevitable the leaked email would lead to Mr Reynolds’ departure, following reports over the weekend that No 10 has already approached three civil servants about replacing him.

One source said: “He’s not a popular guy with officials. He has let down a lot of people. Loads of people at the time, including junior people, said it was a mad idea.”

Another said: “He invited quite a few people and only a third of them turned up. 

“It was ludicrous because we were under really strict orders at the time of how many people could be in rooms at the time. Martin’s job was to enforce how many people could be in at meetings, who needed to be on Zoom.

“He was supposed to be the gatekeeper on Covid awareness. It was extremely out of step. For junior people it was a signal that we can crack on now.”

However, speaking earlier in the day, the Prime Minister’s spokesman denied that Mr Reynolds was being moved to another post – telling reporters: “The Prime Minister has full confidence in his team. There is no change in that post.”