‘We thought it was OK because Boris was there too’: Number 10 staffers blame PM.

One former official described often turning up at No 10 to find it a ‘mess’: ‘There were bottles, empties, rubbish – in the bin, but overflowing – or indeed sometimes left on the table.’

One said a Downing Street security guard, known as a custodian, was mocked when he tried to break up a party.

‘I remember when a custodian tried to stop it all and he was just shaking his head in this party, being like “This shouldn’t be happening,”’ they said.

Sources say Friday drinks had long been a tradition.

‘We saw it as our own bubble,’ one said. ‘Everything just continued as normal. Social distancing didn’t happen. We didn’t wear face masks. It wasn’t like the outside world.’

One said they felt they had the PM’s permission because ‘he was there’, adding: ‘He may have just been popping through on the way to his flat because that’s what would happen.

‘You know, he wasn’t there saying this shouldn’t be happening. He wasn’t saying, “Can everyone break up and go home? Can everyone socially distance? Can everyone put masks on?” No, he wasn’t telling anybody that. He was grabbing a glass for himself.’

One staffer described watching Mr Johnson denying wrongdoing in the House of Commons: ‘We just sort of looked at each other in disbelief like – why? Why is he denying this when we’ve been with him this entire time, we knew that the rules had been broken, we knew these parties happened?’

One said younger members of the team ‘did not think they were breaking the rules at the time because the Prime Minister was at [the events], some of the most senior civil servants in the country were at them – and were indeed organising some of them’.