The prime minister's former top aide has claimed that Mr Johnson misled Parliament after being told a 'Bring Your Own Booze' event on May 20, 2020 would breach coronavirus guidance.
Mr Cummings, who has been strongly critical of his former boss since leaving No10 last November, said he would swear on oath that he warned the PM the proposed drinks party would be against 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 PM waved it aside,” he wrote on his blog.
“The events of May 20 alone, never mind the string of other events, mean the PM lied to Parliament about parties."
Denying the claims, the prime minister said earlier on Tuesday that “nobody told me that what we were doing was against the rules” to the “best of my recollection”.
In parliament last week, Mr Johnson told MPs that he was only in the garden for 25 minutes to thank assembled staff before resuming work in his office, insisting he believed he was at a work event.
But an email, leaked to ITV News - showed 100 staff were invited by the PM's principle private secretary Martin Reynolds to attend "socially distanced drinks" in the Number 10 garden to "make the most of the lovely weather".
This suggests the gathering was not intended to be a work meeting.
At the time, the rest of the country was banned from meeting more than one other person from another household outdoors.
They could only do so in a public place while remaining two metres apart.
Claims by Mr Cummings add to at least 10 allegations of Covid-rule-breaking carried out by members of the Tory party during the coronavirus pandemic.
Despite the government's denial that rules were broken, a leaked video released by ITV News showed Number 10 staff laughing about attending a Christmas party in 2020.
The video sparked swathes of new allegations and a number of resignations.
Now, a growing number of his own MPs - along with opposition members - are demanding Mr Johnson's resignation.
Conservative MPs from the 2019 general election intake are said to have been meeting to discuss getting rid of the PM.
One Tory MP has told ITV News that they believe some 30 letters of no confidence may have been submitted to the PM, around two dozen shy of the 54 needed to trigger a leadership vote.
Our Deputy Political Editor Anushka Asthana has been told that nervous Tory whips are trying to dissuade colleagues from submitting the letters, warning them that there is no going back once they have.
Some of the 2019 intake of Tory MP have said to ITV News UK Editor Paul Brand that even more letters are likely to have been submitted on Tuesday, following an interview in which the PM insisted he was not told about the May 2020 party breaking the rules.
Sources claim the meetings are not yet at the a stage of an orchestrated push to get rid of Mr Johnson, but suggest that tensions are likely to spill over if damning conclusions are produced from Ms Gray's inquiry.
ITV News Political Editor Robert Peston understands that a group of Conservative Red Wall and 2019 MPs have met and reached “a mutual understanding” that Mr Johnson should go.
While some Tory MPs have openly criticised the PM, others are lying low and have said they will not comment until the outcome of the Whitehall investigation, to which Mr Johnson has revealed he has already testified.
The purpose of her probe is to establish "a general understanding of the nature" of gatherings that took place and whether any "individual disciplinary action" should be taken.
The findings of her report, which ministers say will be published shortly, will be made public.