The prime minister's former chief adviser, who is now one of his strongest critics, said he told the leader to get a grip on the “madhouse” when he gave him a warning over one party.
He said on Monday that Mr Johnson “waved it aside” when he raised concerns over principal private secretary Martin Reynolds inviting more than 100 people to an 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.
Mr Johnson responded to the claim by “categorically” denying he had dismissed prior warnings about the gathering.
During a visit to a hospital in north London on Tuesday, Mr Johnson denied he had misled Parliament when he claimed last week that he thought the event was a work gathering and not a party.
“Nobody told me that what we were doing was, as you say, against the rules, that the event in question was something … that wasn’t a work event,” he told Sky News.
“I can tell you categorically, categorically, that nobody told me and nobody said that this was something that was against the rules, it was a breach of the Covid rules, that we were doing something that was not a work event.”
He repeated his claim that when he went into the garden full of staff drinking and eating, “I thought I was attending a work event”. He reiterated his apology to the public “for the misjudgments that I have made, that we may have made in No 10 and beyond” throughout the Covid-19 crisis.
He said he had not seen Martin Reynolds emailed invitation to the event beforehand. “I only saw it the other day,” he said.
Mr Johnson sought to deflect attention away from Mr Cummings' allegations, saying he was awaiting the outcome of Sue Gray's report and would make a statement after its publication.
“My memory of this event, as I have said, is going out into the garden for about 25 minutes for what I implicitly thought was a work event and talking to staff, thanking staff,” he said. He could not recall how many staff were present at the gathering, he said.
On a number of occasions during the televised interview Mr Johnson dodged questions on whether he would resign if he was found to have misled Parliament.
He hung his head in shame when confronted about the parties held in Downing Street on the night before the funeral of Queen Elizabeth's husband Prince Philip.
“I deeply and bitterly regret that that happened and I can only renew my apologies both to her majesty and to the country for misjudgments that were made and for which I take full responsibility,” he said.
Labour's deputy leader Angela Rayner reacted to Mr Johnson's denial by claiming he had been “lying to the British public” and was now only “sorry he got caught".
“Boris Johnson set the rules, he didn’t need anyone to tell him the party he attended broke them. If he had any respect for the British public, he would do the decent thing and resign,” Ms Rayner tweeted.
On Conservative MP predicted Mr Johnson had just “a week” left in office before the scandal results in his political downfall.
The latest allegation came after Education Secretary Nadhim Zahawi denied that a flurry of new policies being announced were an attempt to save Mr Johnson from being ousted as prime minister amid calls for his resignation, including from six MPs from his own party.
In an updated blog post on Monday, Mr Cummings said he had warned Mr Reynolds that his invitation to staff “broke the rules”.
“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.
Mr Johnson insisted he believed it was a work event that could technically have been within the rules.
Before that allegation surfaced, he told the Commons he had been “repeatedly assured since these allegations emerged that there was no party and that no Covid rules were broken”.
Chancellor Rishi Sunak has refused to offer his unequivocal backing to Mr Johnson in his hour of peril.
Mr Sunak, the second most powerful man in British politics, is seen as the most likely successor to the prime minister.
On Tuesday he abruptly cut short an interview when questioned about whether he fully supported Mr Johnson.
Dominic Raab, the Deputy Prime Minister and Justice Minister, said some of his constituents had raised concerns about the “partygate” scandal with him.
“People want to know that there’s not double standards and they want to know that we’re focused on their priorities,” he told Sky News.
“Any perception of double standards is really serious — we understand that frustration. That’s precisely why Sue Gray will look at all of these issues and there will be full transparency and accountability for what happened.”
He said he felt “as frustrated as anyone else” about the parties. “I think double standards is toxic,” he said.
Mr Raab was forced to admit that parties were held in Downing Street on the eve of Prince Philip’s funeral in April last year, while much of the nation was in a period of mourning.
But he stopped short of saying the May 20, 2020 gathering in the Downing Street garden was a party — instead agreeing with the prime minister that it was a work event.
Mr Raab was responsible for running the country for two weeks while Mr Johnson was being treated in hospital for Covid-19.
Asked if he was aware of No 10s alleged drinking culture during this time, he said he was not. But, he said, “it doesn’t surprise me” to hear that people who were working “extremely long hours treated themselves to a glass of wine or beer after a very long week”.
London Mayor Sadiq Khan said he had little doubt that lockdown rules were breached by parties in and backed calls by his Labour colleagues by calling for Mr Johnson to step down.
“Boris Johnson knows what he did and what he didn’t do. We’ve all seen the email, we’ve now seen what Dominic Cummings has said,” Mr Khan told Sky News.
He said the scandal was “great for the Labour party” but the “longer Boris Johnson hangs on” the worse it would be for the UK.
“It’s in the Labour party’s interest for Johnson to stay but it’s in the national interest for Johnson to go,” he said.
Mr Khan said he sees himself as a patriot and therefore believes Mr Johnson “has got to go”.
Complaints about Mr Johnson’s behaviour are continuing among the Conservative party members.
After Mr Johnson apologised in Parliament last week, Science Minister George Freeman sent a letter to his constituents, blaming the prime minister and senior aides for the gatherings during lockdown.
Mr Freeman said people in positions of power “shouldn’t seek to escape public responsibility or accountability”.
“The prime minister and his office should set the highest standards,” he said.
But in a statement posted on social media, Mr Freeman later denied questioning Mr Johnson’s leadership in his letter.
He said he had been very clear regarding the prime minister and that “we need to wait and hear what the official investigation shows”, and the prime minister's response, he said.
A second minister, Maria Caulfield, who works in the Department of Health, voiced dismay at the saga.
She said that whether or not Covid-19 regulations were “technically” breached, “the spirit of the rules” was indeed broken. She told The Times she would “consider what action is needed” after Ms Gray’s report is published.
Meanwhile, reports suggest ministers will announce a series of policy announcements, including putting the military in charge of preventing small boats from crossing the Channel and a freeze on the BBC licence fee, under a campaign to save the prime minister that has been called “Operation Red Meat”.
“Honestly, I don’t recognise that at all,” Mr Zahawi told BBC Breakfast.
“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.
Other touted policy announcements as part of Mr Johnson’s attempted fightback include bids to reduce the National Health Service backlog and a push on the long-awaited levelling-up white paper.
But Mr Zahawi said the policies were “on the list because these are the government’s manifesto”.
He told Sky News it would be a “good idea” to have a “single command and control” to tackle Channel crossings.
“And that includes not just naval vessels but all other vessels, including Border Force, so that you actually have a coordinated operation in terms of the small boats,” he said.
He said the government wanted to “go after the illegal smugglers who are putting these people’s lives at risk”.
But when challenged on the fact the smugglers are not the ones on the boats, he said: “Well, they’re the ones we want.”
Shadow culture secretary Lucy Powell told BBC Radio 4’s Today programme: “Let’s not pretend that this is anything other than it is, which is a pretty obvious dead cat strategy from the government to distract from the totally disastrous leadership context that the prime minister is facing at the moment.”
Mr Zahawi insisted Mr Johnson would stay in his post after further allegations of parties were reported.
Asked three times on Today if the prime minister was safe in his job, he said: “Yes, he is, because he’s human and we make mistakes.
“And, actually, he came to the dispatch box and apologised and said he will absolutely submit himself to Parliament, because that’s our parliamentary democracy.”
Senior official Ms Gray has reportedly questioned Mr Johnson.
Mr Zahawi said she must be allowed to carry out her inquiry after the prime minister had “submitted himself to that investigation”.