The Prime Minister’s former chief adviser said he was prepared to “swear under oath” that Mr Johnson was lying when he told MPs he had not known beforehand about the “bring your own bottle” event on May 20 2020.
In the Commons last week Mr Johnson admitted spending 25 minutes at the gathering saying he had believed “implicitly” that it was a work event.
But in his latest salvo at the man he once worked for, Mr Cummings said both he and another senior official had challenged Martin Reynolds, the Prime Minister’s principal private secretary who sent out invitations to around 100 staff, as to whether it was within the rules.
He said he believed that Mr Reynolds then checked with Mr Johnson who had agreed the event should go ahead.
When he tried to raise the issue with the Prime Minister directly he said his objections were brushed aside.
“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,” Mr Cummings wrote in his blog.
He added: “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.”
In response a No 10 spokesman said Mr Johnson had apologised to the House and that he “believed implicitly” that he had been attending a work event.
“It is untrue that the Prime Minister was warned about the event in advance,” the spokesman said, adding Mr Johnson would be making a further statement when Sue Gray, the senior civil servant investigating partying in Whitehall during lockdown, has completed her inquiry.
The allegations will potentially add to the pressure on Mr Johnson, who is facing calls to resign from some Tory MPs amid widespread voter anger at reports that Covid rules were flouted by No 10.
Allies of the Prime Minister have been mounting a fightback, dubbed operation red meat, with a series of populist policies, freezing the TV licence and putting the Royal Navy in charge of operations to prevent migrant boats crossing the Channel in an attempt to regain support.
Labour deputy leader Angela Rayner said Mr Johnson did not need to wait for Ms Gray’s report and called on him to step down.
“Boris Johnson was warned that the party on May 20 2020 was breaking the rules and yet he not only dismissed those concerns, but attended himself,” she said.
“Instead of hiding behind internal inquiries or technicalities, we need the Prime Minister to tell the truth pure and simple.
“Boris Johnson has failed to show leadership and it’s the British public who have suffered.
“He must resign.”