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.
https://www.bbc.co.uk/news/uk-politics-61566410