The former chief adviser said on Monday Mr Johnson “waved it aside” when he raised concerns over principal private secretary Martin Reynolds inviting more than 100 people to a “bring your own booze” event in the No 10 garden on May 20 2020.
Mr Cummings said regarding that day alone, “never mind the string of other events”, the Prime Minister “lied to Parliament about parties” by insisting he had been assured no events had taken place that would have broken coronavirus rules.
“Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened,” he said.
The fresh allegation came after Education Secretary Nadhim Zahawi insisted a flurry of eye-catching new policies being announced were not an attempt to save Mr Johnson from being ousted as Prime Minister amid calls for his resignation, including from six Tory MPs.
In an updated blog-post on Monday, Mr Cummings said he had warned Mr Reynolds that his emailed invite to staff “broke the rules”.
“Amid discussion over the future of the Cabinet Secretary and PPS himself, which had been going on for days, 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 former adviser wrote.
“The PM waved it aside.
“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.”
Mr Johnson admitted attending but insisted he believed it was a work event which could “technically” have been within the rules.
Before that allegation surfaced, he had told the Commons that he had been “repeatedly assured since these allegations emerged that there was no party and that no Covid rules were broken”.
Meanwhile, reports have suggested that ministers were announcing a series of policy announcements, including putting the military in charge of preventing small boats from crossing the Channel and a freeze to the BBC licence fee, under “Operation Red Meat” to save the Prime Minister.
Mr Zahawi told BBC Breakfast: “Honestly, I don’t recognise that at all.”
He added: “Government doesn’t operate like that.”
However, Culture Secretary Nadine Dorries said the next announcement about the BBC licence fee “will be the last”, reopening the debate over the corporation’s future.
And it was reported that Home Secretary Priti Patel is set to announce within weeks that the Royal Navy will be brought in to spearhead controversial “pushback” tactics to turn away boats carrying migrants across the Channel.
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Other touted policy announcements as part of Mr Johnson’s attempted fightback include bids to reduce the NHS backlog and a push on the long-awaited Levelling Up White Paper.
But Mr Zahawi said the policies are “on the list because these are the Government’s manifesto”.
Senior official Sue Gray, who is investigating a number of possibly rule-breaking events, has questioned Mr Johnson, according to reports.