The candidate was Stanley Johnson, who later failed to get elected.

Sky News approached Stanley Johnson for comment in relation to the allegation.

He said: "I have no recollection of Caroline Nokes at all - but there you go. And no reply... Hey ho, good luck and thanks."

Asked about the claims, Sir Keir told a news conference: "The allegations are serious and they need to be fully investigated. I don't think at this stage it's for me to say what should happen as a result.

"But it takes guts and bravery to come forward, to make allegations like this.

"They now need to be fully investigated, either by the Conservative Party or by the criminal authorities.

"But those allegations having been made, there now needs to be an investigation into them."

The comments by Ms Nokes were made in the course of a cross-party panel discussion, hosted by Sky News, between four prominent female MPs about how to confront violence against women in the wake of the murder of Sarah Everard.

Downing Street earlier refused to comment.

Asked about the allegation at a regular briefing with journalists, Boris Johnson's spokesman said he was "not going to be drawn into individual cases".

The spokesman said he had not spoken with the PM about the claim, adding: "It wouldn't be one for me.

"I'm not going to be drawn into specific allegations against a private individual.

"Of course we would want anyone in any circumstance who feels they have been a victim of any kind of harassment to come forward and report them to the appropriate authorities."

Ms Nokes tweeted on Tuesday: "Just wanted to thank the sisterhood for the solidarity and support today. You know who you are and you're amazing."

Speaking to Sky News earlier, security minister Damian Hinds said an investigation will be carried out into allegations made against the PM's father "if that's the appropriate course of action".

Labour's shadow home secretary Nick Thomas-Symonds told Kay Burley that "any victim who comes forward we have to treat what they say very sensitively, very carefully".