The former chief adviser to Boris Johnson said on Friday he warned at the time that it “seemed to be against the rules and should not happen” but was told it went ahead after he was ignored.
He said that he wrote the warning in an email that could be discovered by senior civil servant Sue Gray as she investigates a string of allegations of rule-breaking parties in Downing Street as the nation social distanced to slow the spread of coronavirus.
Ms Gray replaced Cabinet Secretary Simon Case in leading the Whitehall investigation after allegations emerged of an event taking place within his own office.
In a lengthy blog post on Friday, Mr Cummings insisted there was nothing “illegal or unethical” about a gathering in the Downing Street garden, where staff ate cheese and drank wine during the first lockdown, that was made public after an image was leaked.
However, he wrote: “On Wednesday 20 May, the week after this photo, a senior No 10 official invited people to ‘socially distanced drinks’ in the garden.”
Mr Cummings said he and at least one other special adviser “said that this seemed to be against the rules and should not happen”, adding that he issued the warning “in writing so Sue Gray can dig up the original email”.
“We were ignored. I was ill and went home to bed early that afternoon but am told this event definitely happened,” he continued.
“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.
“In my opinion it would not be fair for most officials who went to the garden for drinks on 20 May to be punished because, given the nature of the invitation, a junior official would be justified in thinking ‘this must somehow be within the rules or X would not have invited me’.
“Other than the 20 May, I’m not aware of events in No10 that ‘broke the rules’ while I was there.”
Mr Cummings appeared in the photo leaked to the Guardian and dated May 15 2020. So did the Prime Minister, then fiancee Carrie and 17 staff members and Mr Johnson’s principal private secretary Martin Reynolds.
On the table in front of Mr Johnson were bottles of wine and a cheeseboard.
Mr Cummings wrote that they had been in meetings that day before “someone brought a bottle of wine out to the table. It may have been Martin but I think it was the PM himself who went inside as I was packing stuff up and brought out wine.
“The scene on the terrace was in no sense a ‘party’ or ‘organised drinks’.”
Mr Cummings said that No 10 staff were “encouraged” to meet in the garden between April and August because it was “safer” during the pandemic.
He said Deputy Prime Minister Dominic Raab made the “obviously false” argument to the public that the photo was taken outside normal hours, arguing staff members often work past midnight on Fridays.