The Health Secretary said the probe into allegations of a Number 10 Christmas party could be widened to include other claims, as the Conservatives admitted a rule-breaking gathering took place at their headquarters.

Sajid Javid said he has been given “assurances” by “senior” officials that no Covid rules were broken by Downing Street staff on December 18, but that it is for Cabinet Secretary Simon Case to use his investigation to “get to the bottom” of whether lockdown-busting events were held last year.

The Cabinet minister told LBC: “If the rules were not broken, then a party could not take place.

“But having this investigation, having the Cabinet Secretary look into this, is the right response because this is the individual who can talk to anyone, can get the data, the evidence together and establish the facts.”

It came as the Tories confirmed an event organised by Shaun Bailey’s failed London mayoral campaign took place in the party’s Westminster headquarters on December 14 while the capital was in Tier 2 restrictions.

According to the Times, the “raucous” party took place in the HQ’s basement, was attended by No 10 aides and featured dancing and wine-drinking into the early hours despite indoor social mixing being banned at the time.

Mr Javid said he has no knowledge of that event, but suggested the head of the Civil Service could choose to delve into what happened as part of his investigation.

“I do know that Simon Case is not just looking at a particular date,” Mr Javid said. “He is free to look at whatever dates he wants to consider.”

Mr Case’s inquiry was ordered by the Prime Minister after a leaked video emerged showing Downing Street aides laughing about a “fictional” party at Number 10 in December 2020.

During a press conference on Wednesday, Boris Johnson also appeared to concede that the Cabinet Secretary would be able to go further than just reviewing the events of December 18 and “look at other things”, with other alleged gatherings in Downing Street over the festive period last year.

Allegra Stratton became the first casualty of the leaked footage when she, in an emotional statement to the press outside her north London home, stood down on Wednesday.

The Health Secretary said he is “not quite sure” why Ms Stratton, who had been Boris Johnson’s spokeswoman before moving to work on the Cop26 climate conference, chose to quit, but he suggested it could have been because she was “embarrassed” by the video.

The footage showed aides laughing about an alleged social event while rehearsing for subsequently-ditched televised press briefings in No 10’s £2.6 million media room, and after it emerged on Tuesday night the Government opted not to put Mr Javid up for Wednesday’s morning broadcast interviews as initially planned.

Asked about that decision, he told Sky News: “I didn’t appear because I saw that video – it upset me, it upset a lot of people I think across the country, it upset the Prime Minister.

“It wasn’t about refusing (to go on air) – I spoke to my colleagues in No 10 and we actually agreed it is best to take some time to respond to the video in the way that the Prime Minister has now, by ordering an investigation by the Cabinet Secretary.”

Pressed on whether he thought an event did take place last December in Downing Street, he added: “The truth is, I don’t know.

“I have received assurances, like the Prime Minister has, that all rules were followed throughout the pandemic at all times.”

Labour deputy leader Angela Rayner said the Cabinet Office probe should be “very easy and quick to do” as she called for those involved in the party to be “dealt with”.

Her comment came as Ed Oldfield, a broadcast adviser to the Prime Minister who was heard asking Ms Stratton a practice question about a rumoured Downing Street party in the video obtained by ITV News, avoided answering whether he plans to resign when confronted by Sky News on Thursday.

Ms Rayner told BBC Radio 4’s Today programme: “There was over 40 people alleged to be at one of the parties doing secret Santa on WhatsApp, so that evidence should be collated and should be given to the Met Police.

“It’s not one rule for them and one rule for everyone else. They’re not above the law. They should be dealt with.”

The Metropolitan Police acknowledged it had received “a significant amount of correspondence” relating to the alleged breaches in No 10 in the run up to Christmas last year but said they do not “provide evidence of a breach” of Covid rules.

But the force did leave open the possibility of further inquiries by saying it would consider “any evidence” that emerges in the Case inquiry.