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 garden at Downing Street during lockdown.
Mr Cummings said regarding that day in 2020 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.
In the blog-post, 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."
The Prime Minister has faced calls to resign over the Partygate scandal that saw Mr Johnson forced to apologise and admit he attended the event at PMQs last week with anger growing among grassroots Conservatives.
He has argued he believed it was a work event that could “technically” have been within the rules.
Senior civil servant Sue Gray is investigating a number of possibly rule-breaking events and has questioned Mr Johnson, according to reports.
If it is established that Mr Johnson knew in advance about the party and lied to parliament in saying he believed it was a “work event”, his position would likely become untenable.
Mr Cummings says that, after Mr Reynolds sent the invitation “a very senior official replied by email saying the invite broke the rules”.
Mr Cummings claims Mr Reynolds told him he would “check with the PM if he’s happy for it to go ahead”, on 20 May 2020.
“I am sure he did check with the PM. (I think it very likely another senior official spoke to the PM about it but I am not sure)”.
Mr Cummings wrote on his substack blog: “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.
“On Wednesday 20 May, a senior No10 official invited people to ‘socially distanced drinks’ in the garden.
“I and at least one other spad (in writing so Sue Gray can dig up the original email and the warning) said that this seemed to be against the rules and should not happen.
“We were ignored. I was ill and went home to bed early that afternoon but am told this event definitely happened.
“In my opinion the official who organised this should anyway have been removed that summer because of his failures over covid. I said this repeatedly to the PM. The PM rejected my argument.”
It came as Conservative Party chairman Oliver Dowden said there are plans to “address the kind of culture that has allowed” alleged rule breaches.
This will include a crackdown on drinking in Downing Street, The Sunday Times reported.
On Monday, the Prime Minister’s official spokesman said Mr Johnson will set out his changes after senior civil servant Sue Gray publishes her inquiry into allegations of partying.
He told reporters: “I think I’d point back to what the Prime Minister’s talked about, that there’s a shared recognition that No 10 should be held to the highest standard.
“There are things clearly we didn’t get right and we need to take responsibility for those. But beyond that, I think it’s right to let the report be published and then you’ll hear more from the Prime Minister.”
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.
Education Secretary Nadhim Zahawi told BBC Breakfast: "Honestly, I don't recognise that at all."
He added: "Government doesn't operate like that."
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.
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".
Speaking on Sky News, he said 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 co-ordinated operation in terms of the small boats," he said.
He said the Government wants to "go after the illegal smugglers who are putting these people's lives at risk".
But when told those are not the ones on the boats, he added: "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 will stay in post after further allegations of parties emerged.
Asked three times on Today whether the Prime Minister is safe, he said: "Yes, he is, because he's human and we make mistakes.
"And, actually, he came to the despatch box and apologised and said he will absolutely submit himself to Parliament, because that's our parliamentary democracy."
Mr Zahawi said Ms Gray must be allowed to carry out her inquiry into reports of coronavirus restriction-breaching events in Westminster after the Prime Minister had "submitted himself to that investigation".