Insiders who attended events at Downing Street during lockdown have told the BBC how staff crowded together and sat on each other's laps and how party debris was left out overnight.
For the first time, insiders who were at some of the events have told BBC Panorama in detail what they saw.
They describe arriving for work the morning after a get-together to find bottles lying around parts of the building, bins overflowing with rubbish and empties left on the table.
They also tell of events with dozens of staff crowded together, and parties going so late that, on occasion, some ended up staying in Downing Street all night.
And they say staff mocked others who tried to stop what was going on.
The prime minister's official spokesman said that Boris Johnson took revelations about what happened in Downing Street during lockdown "very seriously".
He said that the interim report by Sue Gray "raised some of these challenges" and that "wholesale changes" in how No 10 operated were made as a result, adding there were "further changes to come".
The accounts come a day before the senior civil servant Ms Gray is expected to deliver her report on lockdown parties in No 10.
Last week, the Metropolitan Police concluded its own investigation into rule-breaking, after issuing 126 fines - including one for the prime minister for attending a birthday party in June 2020.
However, both the police and the prime minister are facing fresh questions after ITV News obtained pictures believed to show the prime minister at a leaving party for his communications chief Lee Cain on 13 November 2020.
Speaking anonymously, three insiders have opened up about a world behind No 10's famous front door where the lockdown rules the country was living by were routinely ignored, and socialising was regular, with, they felt, the prime minister's implicit permission.
One staffer describes director of communications Lee Cain's leaving do, the event on 13 November 2020, where the prime minister has been pictured raising a glass, but for which he has not been fined.
Others have been judged to have broken the law for being there and received penalties.
Mr Johnson attended and made a speech to thank Mr Cain, but as the party developed "there were about 30 people, if not more, in a room. Everyone was stood shoulder to shoulder, some people on each other's laps…one or two people."
At the party on the eve of Prince Philip's funeral on 16 April 2021, they portray a "lively event... a general party with people dancing around".
The gathering becoming so loud that security guards in the building told them to leave the building and go into the No 10 grounds.
"So everyone grabbed all the drinks, the food, everything, and went into the garden," one source says.
"We all sat around the tables drinking. People stayed the night there."
They now concede what went on was "unforgivable".
The insiders admit that events were routine.
"They were every week," one says. "The event invites for Friday press office drinks were just nailed into the diary."
The invitation was known as "WTF" - meaning "Wine-Time Friday" and a reference to a less polite acronym.
The drinks were often scheduled in No 10 for 4pm. Sources say Friday drinks had been a tradition in Whitehall for some time.
But drinking wasn't limited to Fridays. One former official describes turning up at work in No 10 often to find "A mess! There were bottles, empties, rubbish - in the bin, but overflowing - or indeed sometimes left on the table."
Nearly six months since the allegations about the parties first emerged, it is still almost impossible to believe that socialising was taking place on a regular basis in the buildings where the rules that stopped the rest of the country doing so had been set.
It's clear too that some staff were worried about what was going on, describing the "foolish" now notorious BYOB - bring-your-own-bottle - email sent by the prime minister's top civil servant, Martin Reynolds.
Instant messages were flying around between staff questioning what on earth was going on.
But a former staffer says how difficult it felt to raise concerns.
Watch Laura Kuenssberg's Panorama, Partygate: Inside the Story, on BBC Two at 19:00 BST or later on the BBC iPlayer
Another insider describes how a custodian, a Downing Street security guard, was mocked when they tried to stop a party in full flow.
"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'."
"People made fun of him because he was so worked up that this party was happening and it shouldn't be happening."
How did it happen then, when the rest of the country was living under strict lockdown?
All three paint a picture of a Downing Street as a parallel universe. "We saw it as our own bubble" where the rules didn't really apply, says one.
"Everything just continued as normal. Social distancing didn't happen. We didn't wear face masks. It wasn't like the outside world."
Another even describes the events as a "lifeline" for staff who were working long hours, especially if they lived alone.
But all three point to the culture set by the prime minister himself, suggesting he "wanted to be liked" and for staff to be able to "let their hair down".
One suggests they felt like they had the prime minister's permission to socialise even it meant breaking the rules because "he was there."
"He may have just been popping through on the way to his flat because that's what would happen," they add. "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."
Mr Johnson has been fined for attending one event and denies that he broke the law on other occasions, or was aware that rules were being broken in Downing Street.
But it's clear as insiders speak out for the first time that some of those who were working in No 10 at the time find that account hard to accept.
One staffer describes what happened when they watched the prime minister denying, in the House of Commons, that anything had gone wrong.
"We were watching it all live and we just sort of looked at each other in disbelief like - why?" they say.
"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?"
The political consequences of Partygate have been profound for the prime minister.
The government has been battered by the scandal for months. Trust in him has been damaged, and he is facing renewed calls from his party to quit.
He has so far survived, with a majority of his MPs reluctant to act. He only received one fine and his backers believe the public care far more about making ends meet than how many glasses of warm Prosecco the prime minister may or may not have briefly drunk with his staff.
As Downing Street braces for the findings of the official Whitehall inquiry into what happened, which raises the tension again, the personal consequences for those who are caught up in the mess is clear.
Many dozens of staff and former staff have been fined, including two of those who opened up to Panorama, and there is hurt and confusion about how they have been treated.
One former staffer says that particularly 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".
Some feel they have been let down and "subject to this witch hunt" while more senior officials and politicians have found it easier to carry on.
There's embarrassment but sadness at the saga too. "It's been very distressing and shaming and, particularly because that whole period was just quite traumatic, it was really difficult to work on every single day.
"We were learning that people were dying in hospital beds and people were dying needlessly…So it was quite difficult to look back at that period now and think this is what will define that - not the vaccine programme or the food parcels for shielding people.
"It will be 'What were you doing on 20 May in the garden?'"
It is a saga that the government knows many members of the public won't forget.
Mr Johnson's future will be shaped by whether they are willing to forgive.