TV producer Daisy Goodwin has submitted the complaint after accusing the mayoral hopeful of groping her in an alleged incident in 2013, saying she was left "amazed, shocked, and rather humiliated".

Mr Korski has strenuously denied the claim against him and has pledged to stay in the race..

Number 10 said the "serious" allegations "should be dealt with by the appropriate authorities", though they did not make it clear who those authorities were.

Current London Mayor Sadiq Khan - who is set to contest his job next year against the eventual Tory nominee - said: "The Conservative Party will be judged in their response" to the alleged incident.

He told Sky News: "A very serious allegation's been made of sexual assault. I find it astonishing that the Conservative Party are proceeding with the process in the way that they are. It's a question for them not for me."

He added: "I'm quite clear being the mayor of London means making sure that you are a mayor for everyone, and it's for the Conservative Party to answer questions about how they've handled this allegation."

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The allegation was first made in an article for The Times on Tuesday, in which Ms Goodwin wrote that Mr Korski had groped her breast at a meeting in Number 10 when he was working for the then prime minister, David Cameron.

Ms Goodwin confirmed the complaint had been made in a post on Instagram this morning, which contained a screenshot of an email reply from the Cabinet Office which is marked "formal complaint about Daniel Korski".

Speaking to BBC Radio 4's Today programme, she said she was prompted to lodge the official complaint because the government had said they could not investigate until one was made.

She added: "It's quite difficult to make a complaint.

"It's like trying to get through to BT if your phone line has gone down, it's not an easy process."

She said she decided to speak out now because Mr Korski is running for London mayor.

"I was shocked by it - I can't say that I'm a victim, but I was shocked by it," she said.

"And since I wrote my piece, I have been contacted by other women with some very interesting stories which clearly I can't talk about for legal reasons, but I feel entirely justified in having written the piece and naming him."

A number of Tory MP backers of Mr Korski have withdrawn support for his campaign in the wake of the allegation.

Earlier on Wednesday, children and families minister Claire Coutinho told Sky News she had paused her support for Mr Korski and said that any complaint should be investigated "swiftly".

She added: "But at the same time, this is an allegation. He's roundly denied it.

"If there is a complaint in the system, it needs to be followed up swiftly so we can find the facts and see what's happened, but I do think it's a very serious and concerning allegation."

Sky News has been told that education minister Robert Halfon has also paused his backing of Mr Korski.

Downing Street described Ms Goodwin's allegation as "very serious".

Asked if Rishi Sunak believes Mr Korski is a suitable candidate, the prime minister's press secretary said: "As you know there are three candidates and the Prime Minister does not endorse any single one candidate.

"Obviously these allegations are very serious. They are allegations that have obviously been denied by Daniel Korski himself. They should be handled in the proper way."

Mr Korski used an interview with TalkTV on Tuesday to say that he had met with Ms Goodwin, but denied that anything improper had occurred.

He said the allegations against him had been reported to the Conservative Party before he stood as the Tory candidate for the race to replace Sadiq Khan as London mayor in 2024, and will "absolutely" continue his campaign to win the nomination of his party.

Mr Korski's statement said: "I know that simply denying such allegations may not be enough to alleviate the concerns and doubts that might arise in your minds.

"I firmly believe in the importance of empathy, respect, and the wellbeing of every individual within society."