Boris Johnson has full confidence in his director of communications Jack Doyle, Number 10 has said, despite allegations he broke Covid rules at a Christmas party during last winter's lockdown.
But "disciplinary action" will follow if an investigation into the party finds any rules have been broken, a minister told ITV News this morning.
Paul Scully's comments came after ITV News revealed claims Mr Johnson's top adviser was at an alleged party which is reported to have been held in Number 10 on December 18 last year.
Asked if Mr Doyle should be sacked if he was found to have breached Covid rules, Mr Scully said: "If rules were broken then disciplinary action will clearly follow."
An investigation has been launched into the alleged party, along with two other gatherings which were allegedly held by members of the government during last winter's lockdown.
The prime minister ordered the probe, to be carried out by the UK's top civil servant, after ITV News released a leaked video showing Number 10 staff joking about the December 18 party.
Listen to our coronavirus podcast on the inside scoop of how ITV News obtained the leaked video showing senior Downing Street staff joking about a Christmas party
Evidence of any rule breaking uncovered during the investigation will be passed to the police, a minister confirmed on Thursday.
The Metropolitan Police is facing growing calls for it to investigate the alleged party, which would have broken Covid rules at the time, given London was under strict Tier 3 restrictions which outlawed almost all indoor gatherings.
The force has previously said it will not probe claims of rule-breaking as it is not policy retrospectively investigate Covid breaches, but said it was considering what action to take after viewing the leaked video.
Mr Johnson had repeatedly insisted no rules were broken in Downing Street, but the video's release sparked a change of tack.
He apologised for the "sickening" video and asked Permanent Secretary Simon Case to investigate whether any rules were broken.
Labour Leader Sir Keir Starmer said the prime minister is "not fit for office" following the recent revelations.
"This is just the latest allegation of dishonesty from the Prime Minister, we've had lie upon lie in relation to the parties going on in Downing Street.
"The prime minister is not fit for office, and people are paying the price for this."
Pressure is growing on Boris Johnson over the alleged party in Downing Street last Christmas, as ITV News Political Correspondent Carl Dinnen reports
At PMQs on Wednesday, he said: "I can understand how infuriating it must be to think that people who have been setting the rules have not been following the rules because I was also furious to see that clip."
But again said he's been "repeatedly reassured" there was no party.
Those assurances have been thrown into doubt by revelations that Mr Doyle - the prime minister's director of communications, who will have shaped much of the government response to the party allegations - may have attended the bash.
ITV News has been told that not only was Mr Doyle at the party, but he handed out certificates as part of a joke awards ceremony to members of the communications team.
The PM's former press secretary Allegra Stratton, who said in the leaked video that there was "definitely no social distancing" at the party, resigned as a government adviser, saying she would "regret those remarks for the rest of my days".
But Ed Oldfield, another adviser who also featured in the video, and Mr Doyle still remain in their job, despite calls for them to be sacked.
Senior Downing Street staff joke about holding a Christmas party in a mock televised press briefing
Labour's shadow health secretary Wes Streeting said: "I'm sure Boris Johnson is busy planning who he is going to throw under the bus next.
"We've had the resignation of Allegra Stratton, there are questions now about the Prime Minister's communications director, but you know, whether it's Allegra Stratton, Jack Doyle, Dominic Cummings, Gavin Williamson, Matt Hancock, these people who have flagrantly broken the rules and in full view of the public with no accountability, they have one thing in common and that's the man who's appointed them and his judgment.
"And I'm afraid it's his untrustworthy nature, his disorganisation, his dishonesty, which is undermining trust in public health measures."
The PM is also being accused of being dishonest over a separate issue - the funding of his flat's refurbishment.
Labour has accused the prime minister of having lied to the ministerial standards adviser over whether he knew a Tory peer was behind donations for the lavish renovations.
Under what circumstances are Labour calling for Boris Johnson to resign?
Lord Geidt is reportedly considering resigning over allegations he was misled by Mr Johnson during his investigation into the refurbishments costing more than £112,500.
Labour Deputy Leader Angela Rayner said: "We now know that in the days before he imposed the 2021 winter lockdown, the prime minister went from allegedly hosting an illegal party in Downing Street to asking super-rich Tory donors to secretly fund the luxury refurb of his flat.
"Not only has the Conservative Party broken the law, but its Prime Minister has made a mockery of the standards we expect.
"If Boris Johnson refuses a fresh investigation, that standard will be lowered significantly - setting the bar woefully low for our country's public life."