He also claimed loosely that Mr Johnson was aware of other parties which had taken place in 2021 and pictures of the gatherings, though provided no specifics or proof.

In the blog post, Mr Cummings claimed that on May 20 2020 a “senior No 10 official invited people to ‘socially distanced drinks’ in the garden”.

Mr Cummings said he wrote in an email at the time that the gathering “seemed to be against the rules and should not happen”.

The Telegraph has been told by three different sources that the official who issued the email was Mr Reynolds. Downing Street did not deny the claim last night.

The two insiders said they recalled the email being sent directly by Mr Reynolds, although a third said they believed it had been sent by an official on his “behalf”.

The email, which is now understood to have been wiped from No 10’s email system, was said to have angered a number of senior staff in the building, who are said to have raised their concerns with Mr Reynold’s about the “tone” of the invite.

“Various people said ‘Martin, what the f—’,” one source claimed, adding: “people went up to him and asked if it was a good idea.” Another source said of the controversy over the email: “People said ‘are you mad?’”

‘Bring your own booze’

Two sources independently described the email as an invite to a “knees up”, while a third said it was framed as an “informal get-together” to recognise the “hard work” of staff during the pandemic. Officials were encouraged to “bring your own booze”.

“It had a big knees-up sort of tone,” one said. “It was very jarring with what was going on. The tone of that one was very much ‘we should have a party’.”

Officials said that other more informal gatherings in the garden regularly occurred throughout the pandemic, adding that there was a culture that “persisted into 2021”, when the country was exiting the roadmap out of the third lockdown.

One said they recalled receiving regular messages in Downing Street regarding the use of the garden, including reminders to staff to “clear up your beer bottles and pizza boxes”.

“There was a culture of not thinking about what it looked like,” they added.

On the same day, May 20, Oliver Dowden, then Culture Secretary, urged members of the public to continue only meeting one other person outdoors, while adhering to social distancing, as required by the rules at the time.

During the Downing Street press conference, Mr Dowden ran through presentation slides which said the public should “stay at home as much as possible”, to “work from home if you can” and “limit contact with other people”.

“If everyone stays alert and follows the rules we can control coronavirus by keeping the R down and reducing the number of infections.”