An email inviting No 10 staff to “socially distanced drinks” during the Covid-19 pandemic was “totally inappropriate” in its wording, the senior official who sent it has admitted.

Martin Reynolds, who was Boris Johnson’s principal private secretary at the time, emailed staff inviting them to come for drinks in the No 10 garden to “make the most of this lovely weather” on May 20, 2020.

Mr Johnson attended the “bring your own booze” event, at a time when rules and guidance restricted gatherings of more than two people and workplaces were meant to maintain social distancing.

In his evidence to the Privileges Committee, Mr Reynolds said he had not drafted the message, but he did sign off on it.

The email that was sent out said: “After what has been an incredibly busy period we thought it would be nice to make the most of this lovely weather and have some socially distanced drinks in the No 10 garden this evening. Please join us from 5pm and bring your own booze!”

Mr Reynolds told MPs: “With the benefit of hindsight, the language used was totally inappropriate and gave a misleading impression of the nature of the event.

“It was an event held because staff needed a morale boost after an extremely difficult period when all sorts of tensions had begun to surface and I hoped that being thanked by the PM and talking to each other might strengthen their sense of being part of one team.

“The event was not a party in any normal sense of the word.”

He insisted he did not believe the event was against the rules, but the Metropolitan Police issued fines related to it.

Mr Johnson’s communications chief at the time Lee Cain said he raised concerns about the email with Mr Reynolds, warning that it was “somewhat of a comms risk in the current environment”.

Mr Reynolds said he did not believe that Mr Cain had called for the event to be cancelled or suggested it was against the rules but “the concern was that the invitation or a photo of the prime minister with a glass of wine” would leak to the media and “this could be misconstrued”.

It would have been “inconceivable” for the event to have gone ahead if Mr Cain and Mr Johnson’s senior adviser Dominic Cummings had argued against it, Mr Reynolds added.

Mr Cain told MPs it would have been “highly unusual” for him not to have raised concerns with Mr Johnson about the event.

He said he could not remember if he personally had a conversation with the then-prime minister about it, but stated that he told Mr Cummings about his concerns over the gathering.

Mr Cain said he told Mr Cummings about his concerns who “agreed it should not take place and said he would raise the issue with Martin and the prime minister”.

He added: “I do not recall if I personally had a conversation with the PM about the garden party but it would have been highly unusual for me not to have raised a potentially serious communications risk with the PM directly – especially having raised it with his PPS and the matter remaining unresolved.”

To Mr Cain, “it was clear observing all who attended and the layout of the event that this was purely a social function”.

Mr Cummings told the Sue Gray inquiry into partygate that “the idea the PM could have thought this drinks event was ‘work’ is comical, given the tables covered in bottles of drink, everyone standing around drinking”.

He told the Privileges Committee that he told Mr Johnson the event was against the rules and “he should overrule (Mr Reynolds) and stop it”.

He said that as a special adviser he could not give orders to civil servants to cancel the event.

“If I could have issued such orders, this event would not have occurred,” he told MPs.