The Government's internal inquiry into rule-busting Downing Street parties is not expected to be published until late January, the Mirror understands.
Whitehall enforcer Sue Gray, who is investigating the claims, has turned her attention to other gatherings which have come to light since the initial probe began.
The top civil servant is said to be planning to interview staff involved in the No 10 garden party and a gathering in the Cabinet Secretary's private office.
She is already understood to be looking into four events first revealed by the Mirror - three in Downing Street and one at the Department for Education.
Sources suggested it was within her remit to interview Boris Johnson, who was believed to be present at three of the gatherings under investigation.
One source said: "She wouldn't want to be heading up any investigation that looked like it was ducking speaking to any potential witnesses.
"It could raise eyebrows if she doesn't".
The Cabinet Office was unable to say whether the PM, who gave a speech at a leaving do, hosted a festive quiz and was pictured with wine and cheese in the Downing Street garden, all in apparent breach of the lockdown rules at the time, would be interviewed
Cabinet Secretary Simon Case had to recuse himself from leading the investigation after the revelation over the 'Christmas party' event in his office.
The inquiry, which No 10 has said would be completed "quickly and thoroughly", is expected to be made public.
Findings of a separate probe by the PM's ethics advisor Lord Geidt into the refurb of the Downing Street flat are due to be published imminently.
Letters between Mr Johnson and Lord Geidt are expected to be published this afternoon, but Mr Johnson is likely to be cleared again of breaching any ministerial rules over the funding of the redecoration.
Mr Johnson was initially cleared of breaking the rules over the lavish revamp of his No11 flat.
He told Lord Geidt he had no knowledge of payments made by a Tory donor towards the costs until February 2021.
But last month, the Electoral Commission revealed that the PM had sent Tory peer Lord Brownlow a WhatsApp message in November 2020 "asking him to authorise further, at that stage unspecified, refurbishment works on the residence".
