Insiders who were present at Downing Street events during lockdown told BBC that staff crowded together, sat on one another's laps, and how party debris was left behind overnight.
BBC Panorama has been granted details by insiders who witnessed the events for the first time.
They described arriving at work one morning after a party to find bottles scattered around the building, trash everywhere and empty plates.
There were also stories of parties that went on until the early hours and involved dozens of employees.
They also claim that staff ridiculed others who tried stopping the action.
Official spokesman for the prime minister said that Boris Johnson had taken revelations about Downing Street's lockdown "very seriously".
He stated that Sue Gray's interim report "raised some challenges" and that wholesale changes in No 10's operation were made as a consequence. He also said there would be "further changes".
These accounts are available a day before Ms Gray, a senior civil servant, is expected to present her report on lockdown party in No 10.
The Metropolitan Police closed its investigation into rule-breaking last week and issued 126 fines, including one to the prime minister for attending June 2020's birthday party.
ITV News has obtained images that show the prime minster at a party to thank Lee Cain, his communications chief, for leaving on 13 November 2020. This raises new questions about both the police force and the prime minister.
Three insiders spoke anonymously about the world behind No 10. They described a world where the lockdown rules that the country lived by were routinely ignored and socialising was regular with the implicit approval of the prime minister.
One staffer describes Lee Cain, director of communications, leaving do on the 13th November 2020 at which the prime minister was pictured raising a glass but has not been punished.
Others were found guilty of breaking the law and punished.
Mr Johnson was present and gave a speech to Mr Cain. However, the party grew rapidly and there were "about 30 people in a room, if that is not more." All were standing shoulder to shoulder, with some people sitting on their laps, and a few others in the middle of each other.
They portray the party as a lively event... with lots of people dancing at it.
The noise was so strong that security guards had to tell them to get out of the building and enter the No 10 grounds.
One source claims that everyone took all the drinks and food and went into the garden.
"We all sat around tables and drank. Many people stayed there all night."
They have now admitted that what happened was "unforgivable".