Boris Johnson “waved aside” objections and let the No10 party in May 2020 go ahead, his ex-aide claims.
Dominic Cummings says he and others would swear on oath that top officials warned the event was against the rules.
It would mean the PM lied to the Commons when he claimed he thought it was a work event.
Cummings claims he told Boris Johnson “you’ve got to grip this madhouse” after the invite was sent out asking around 100 staff to attend the BYOB party at No10.
He says at least two senior officials told Boris Johnson’s Principal Private Secretary Martin Reynolds to cancel the garden party on May 20, 2020.
Mr Reynolds then allegedly checked with the PM whether the event could go ahead, and was told that it could.
Mr Cummings, a bitter foe of the PM since his departure from No10 in November 2020, also claimed he had challenged Mr Johnson himself.
If true, this means the PM lied to the house in his apology last week.
In his latest blog, Mr Cummings writes: “I said to the PM something like: Martin’s invited the building to a drinks party, this is what I’m talking about, you’ve got to grip this madhouse. The PM waved it aside.”
He said it was “not credible” that Mr Reynolds would have told senior officials he would check with the PM and then failed to do so.
Mr Cummings said: “The events of May 20 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.”
Mr Johnson attended the party, it is understood with his wife Carrie. He sparked widespread disbelief last week at PMQs when he claimed he “implicitly believed” it was work related.
In response to Mr Cummings’ claims, a No10 spokesman said last night: “It is untrue the Prime Minister was warned about the event in advance. As he said earlier this week, he believed implicitly that this was a work event.
“He has apologised to the House and is committed to making a further statement once the investigation concludes.”
But Mr Cummings’ allegations will increase the jitters of Tory MPs.
Senior Tory MP Steve Baker said his constituents were “about 60 to one against the PM” over the parties row.
The MP said “rule-makers must obey the rules”, but he said he would wait for Whitehall enforcer Sue Gray’s report before taking any action.
Several MPs told the Mirror they had received hundreds of emails from furious constituents about the PM’s apparent rule-breaking.
Others said their local associations had urged them to put in a letter to kick off a vote of no confidence in Mr Johnson.
One Tory told the Mirror: “The PM is fatally damaged, way beyond repair. Activists are refusing to campaign at the local elections until he is gone.”
Sue Gray, whose report is expected later this week, is understood to have interviewed Mr Johnson, and No10 sources claim he has “shared what he knows”.
Mr Johnson has launched “Operation Red Meat” to try to save his skin with policy announcements, including scrapping the BBC licence fee.
Mr Johnson is also planning to overhaul his top team in “Operation: Save Big Dog”, with Mr Reynolds and chief of staff Dan Rosenfield in the line of fire.
Six Tory MPs have demanded the PM resign, while many more are waiting for Ms Gray’s report before they act.
Education Secretary Nadhim Zahawi insisted the PM was safe. He said: “He’s human and we make mistakes.”
Ominously for Mr Johnson and his supporters, Mr Cummings claimed: “There are many other photos of parties after I left yet to appear.”
