Downing Street officials were “p***ing themselves” when top civil servant Sue Gray invited them for formal interviews on No10 lockdown parties held last year.
Ms Gray took over the investigation into the parties held last December after Simon Case, the Cabinet Secretary, was revealed to have had a party of his own.
Since taking the lead, Ms Gray is said to have sent more than a dozen emails about the events, The Times reported.
The party that took place days before last Christmas on December 18 is expected to be at the heart of her investigation.
The Mirror revealed members of Boris Johnson's top team held their own festive bash in Number 10, knocking back glasses of wine during a Christmas quiz and a Secret Santa while the rest of the country was forced to stay at home.
A source told the Mirror, "it was a Covid nightmare" as people were said to have been crammed "cheek by jowl" into a medium-sized room.
Jack Doyle, the Prime Minister's director of communications is said to have addressed staff at the party and even handed out awards.
The PM's spokeswoman, Allegra Stratton, quit after being filmed joking about it with fellow aides at a mock press conference.
Among the lockdown-breaching parties said to have taken place last Christmas, Mr Case's office held a virtual quiz on December 17, starting at 5.30pm and lasting an hour.
Alcohol and snacks were enjoyed by those present, which were provided by staff themselves.
Mr Case spoke to staff briefly at the end of the event, and then left.
After a leaked photo showed Downing Street staff eating cheese and drinking wine in the No 10 garden during the first lockdown, the PM said it showed “people at work, talking about work”.
The photo, obtained by the Guardian, showed Mr Johnson, his then-fiancee Carrie, and 17 other staff members in the garden on May 15, 2020, with bottles of wine and a cheese board on a table in front of the Prime Minister.
Conservative Chairman Oliver Dowden has said: “I'm confident that that inquiry will vindicate the Prime Minister's assertion that everything that happened was within the rules.”
