Downing Street has stood by the official who sent the email with an invite to the “BYOB” gathering.
A spokesman for the prime minister said they had full confidence in his principal private secretary Martin Reynolds.
No10 said Reynolds “continues in his role” and has the full confidence of Boris Johnson.
It comes after an email from him to 100 colleagues was leaked to ITV News, in which he said: “After what has been an incredibly busy period it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening.
“Please join us from 6pm and bring your own booze!”
ITV has alleged that 40 people attended the gathering — including the prime minister and his wife Carrie Johnson — on May 20, 2020 when the country was in national lockdown.
Earlier the sam day, the then culture secretary Oliver Dowden told the public they could only meet with one other person outside of their household in a public outdoor space, provided they kept two metres apart.
Police have since confirmed they are “in contact with the Cabinet Office” following the reports of the drinks party, which is also the subject of an investigation by civil servant Sue Gray.
The prime minister previously refused to confirm whether he attended the event or not and would only say that it was subject of a “proper” investigation by Gray.
And Downing Street repeatedly refused to respond to questions on whether Johnson attended the party, instead saying it was a matter for Gray’s investigation.
Asked why the prime minister could not confirm himself whether he was at the event, Johnson’s official spokesman said: “Whilst this investigation is ongoing I can’t comment on the reports and claims including those we’ve seen today.
“It wouldn’t be appropriate to do so.”
Pressed on whether Johnson’s silence risked creating the impression he was guilty, the spokesman said: “I think what everyone wants to do is establish the facts through this independent review and for those facts to be set out clearly once that work is concluded.”
And asked whether he had received assurances that there was no party on May 20 — as had been the case with previous allegations — he replied: “I’m simply going to have to repeat myself. I’m not going to be commenting on those claims and other claims regarding different dates while this work is ongoing.”