The inquiry ordered by Boris Johnson from cabinet secretary Simon Case will look only at allegations of a Downing Street party on 18 December last year and not into wider claims of gatherings on other dates, No 10 has said.
And a Downing Street spokesperson said that the civil service chief has been given no deadline to deliver his verdict on the explosive claims.
While No 10 said the prime minister wanted answers as quickly as possible, the spokesperson said that it was for Mr Case to decide how long his probe should take.
The Daily Mirror article which initially reported that a Christmas party took place in No 10 on 18 December also gave details of a second bash on 27 November to say farewell to a departing aide, which Mr Johnson is alleged to have attended.
And a third party allegedly took place on 13 November, the day of the departure of Mr Johnson’s former top aide Dominic Cummings.
Asked whether Mr Case would be free to inquire into these allegations, a No 10 spokesperson said: “He has been asked to establish the facts of any events on the 18th.”
Downing Street refused to say whether Mr Case himself was at any party at No 10 on the evening of the 18 December.
And spokespeople declined to confirm whether the prime minister himself now believes that a party in fact took place.
They would also not say whether Downing Street stands by official statements made over the last week that no party took place and that Covid rules were observed at all times.
“You have our previous statements,” said a source. “Now the prime minister has asked the cabinet secretary to establish the facts.
“While that is ongoing it wouldn’t be appropriate for me to discuss details.”
The PM’s press secretary said that ministers had been pulled from scheduled TV and radio interviews on Wednesday morning because Mr Johnson wanted to announce the Case inquiry directly to MPs himself.
But she said that the usual practice of ministers being made available for a morning round of interviews will be resumed on Thursday.
