Exposing a damning culture of regular rule-breaking, three anonymous individuals told BBC Panorama the Prime Minister never sought to break up the scenes when returning to his flat.
Their claims pile further pressure on Mr Johnson ahead of the publication of the Gray inquiry into “partygate”, which No 10 expects on Wednesday.
It came on the same day the acting head of the Metropolitan Police was asked by London Mayor Sadiq Khan to explain the force’s decisions after Mr Johnson was fined only once despite fresh pictures having been published of him drinking at a gathering.
One insider said they felt they had the permission of the Prime Minister as he was not telling them to break up the scenes when returning to his flat.
“No, he wasn’t telling anybody that – he was grabbing a glass for himself,” they said.
Asked about the pictures of Mr Johnson raising a glass while surrounded by colleagues and bottles of wine, one of the insiders said there were even more people present than pictured.
A witness said: “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.”
The insiders said the events were “every week”, with invitations for press office drinks listed in the diary as “Wine-Time Friday”.
One former official described often turning up at No 10 to find it “a mess”.
They added: “There were bottles, empties, rubbish – in the bin, but overflowing – or indeed sometimes left on the table.”
“Unforgivable” scenes were also described at the party on April 16 last year, which was the eve of the Duke of Edinburgh’s funeral.
They described a “lively event … a general party with people dancing around” that became so loud that security guards told them to go into the No 10 grounds.
The witness said: “So everyone grabbed all the drinks, the food, everything, and went into the garden.
“We all sat around the tables drinking. People stayed the night there."
The trio claimed things were so chaotic, party debris was left overnight for other staff to discover.
One said a Downing Street security guard, known as a custodian, was mocked when he tried to break up a party.
They said: “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’.”
Meanwhile, No 10 was forced to deny Mr Johnson urged Ms Gray to drop plans to publish her report in a private meeting earlier this month.
It has been reported the Prime Minister suggested to the senior civil servant there was no longer any point in releasing her findings as the facts were “all out there”.
Rather than an outright denial, Mr Johnson’s spokesman said he did not recognise “that characterisation” of the meeting.
He said: “This was a legitimate meeting about the process [of publication] rather than the contents of report.
“The Prime Minister did not ask her to drop the report or not proceed with the report. It was the Prime Minister who commissioned the report. He wants the report to be published.”
Scotland Yard was also facing calls to explain why Mr Johnson was not fined over the farewell event for his communications chief Lee Cain on November 13, 2020, when photos showed the Prime Minister, drink in hand, by a table strewn with food and wine bottles.
The London Mayor wrote to Sir Stephen House to seek answers about the Met’s decisions in individual cases in the Downing Street investigation.
Mr Johnson received a fixed-penalty notice (FPN) over a birthday party in the Cabinet Room in June 2020, but was told he would face no further action over other gatherings covered by the Metropolitan Police’s Operation Hillman inquiry.
A spokeswoman for Mr Khan said: “Sadiq has today written to the acting Commissioner of the Met to seek a detailed explanation of the factors which were taken into account by investigating officers when decisions were made about whether to take action in individual cases in the Downing Street partygate investigation.
“He has asked them to take steps to also reassure Londoners by making this explanation to them directly, because he is concerned that the trust and confidence of Londoners in the police is being further eroded by this lack of clarity.”
Earlier, transport secretary Grant Shapps insisted Mr Johnson did not believe he was at a lockdown-breaking party in the images showing him raising a glass and surrounded by colleagues and wine bottles.
Mr Shapps said he was “angry” to see the photographs, but suggested the Prime Minister may not have been fined over the event because he left the leaving do “pretty quick”.
Mr Johnson is expected to address the Commons and host a Downing Street press conference after the Gray report is published.
He will also meet with the 1922 Committee of backbench Tories in a bid to shore up support.
Scottish Tory leader Douglas Ross demanded the Prime Minister explain why he believed his behaviour was “acceptable” when most people would think the pictures showed “unjustifiable and wrong” behaviour.
Conservative MP David Simmonds said he was awaiting Ms Gray’s report, but that it would be “very difficult” for Mr Johnson to explain how he did not mislead Parliament.