An email sent to ITV News shows one of the PM’s aides inviting around 100 people to attend a ‘socially-distanced drinks’ in the Number 10 garden and a request to ‘bring your own booze’.
It’s also been alleged that the event was attended by the PM and his wife on May 20, 2020.
Mr Milband, speaking to the BBC’s Today programme, said the event was ‘the most clear violation of the rules’.
On the same day, the government announced that people were allowed to meet one person outdoors – provided they stayed two metres apart.
Number 10 has said they would not comment on the story due to the ongoing inquiry carried out by senior civil servant, Sue Gray.
The email was sent by the Prime Minister’s Principal Private Secretary Martin Reynolds which read: “After what has been an incredibly busy period we thought it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening.
“Please join us from 6pm and bring your own booze!”
The email has sparked mass anger from opposition politicians and key workers.
Doncaster North MP, Ed Miliband said: “These are incredibly damning revelations and the reason this is so damning is because this was an organised party, no ifs, no buts.
“It was clearly the most clear violation of the rules, it was on the day the Cabinet Minister Oliver Dowden said you were only allowed to meet one person outdoors, they were the rules in place.
“But crucially, Number 10 has not denied that the Prime Minister was at the party. It’s all very well that we are having Sue Gray’s inquiry, but the Prime Minister cannot run and he cannot hide – he’s got to answer (for it).
“If I went to the party, I would know if I went to the party he’s got to explain was he at the party and how he can possibly justify all of the things he said in the House of Commons, that no rules were broken and he did nothing wrong – he is going to have to answer.
“I’m afraid it speaks to a rotten culture at the heart of this government and the rotten culture begins with the person in charge.”