Boris Johnson’s former chief adviser has said the prime minister knew in advance about a Downing Street drinks party during the first coronavirus lockdown and agreed that it could go ahead.

In an updated post on his Substack page, Dominic Cummings said he raised his concerns about the event in the Number 10 garden in May 2020 directly with the PM and would “swear under oath this is what happened”.

“PM was told about the invite, he knew it was a drinks party, he lied to parliament,” he wrote on Twitter, posting a link to the latest entry.

Mr Johnson’s principal private secretary Martin Reynolds emailed officials with an invite to the event on 20 May, suggesting attendees “BYOB” (bring your own bottle, or booze).

A second Downing Street source has told Sky News they believe the PM was warned about the gathering before it took place.

They have told Sky’s political editor Beth Rigby that, per Mr Cummings’ blog post, Mr Reynolds was warned about the event after sending out the email invitation, and believes Mr Johnson was informed.

But Downing Street said earlier on Monday it was not true that Mr Johnson was warned about the gathering in advance and repeated the PM’s assertion to MPs last week that he “implicitly” believed it was a work event.

Number 10 has repeated this denial in the wake of Mr Cummings’ latest post.

Mr Cummings left Number 10 in November 2020 and has since been a persistent critic of Mr Johnson.

“MPs should focus on the basics,” he wrote on his Substack.

“The PM’s PPS [principal private secretary] invited people to a drinks party. The PPS was told to cancel the invite by at least two people.

“He checked with the PM whether the party should go ahead. The PM agreed it should.

“They both went to the party. It was actually a drinks party.

“The PM told MPs repeatedly that he had no idea about any parties.

“The events of 20 May alone, never mind the string of other events, mean the PM lied to parliament about parties.

“Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened.”