A cabinet minister has said she “would have probably slapped” Stanley Johnson if he inappropriately touched her in the way alleged by senior Conservative MP Caroline Nokes.

International Trade Secretary Anne-Marie Trevelyan said the move would have been her “instinctive response” and that Ms Nokes demonstrated “great personal restraint if she quietly moved away”.

Ms Trevelyan added that – if the allegations are accurate – Mr Johnson’s actions were “not acceptable”.

“I would always call anyone who feels under that sort of pressure to speak up, and we will always stand by them,” the cabinet minister told Kay Burley.

Ms Nokes, a former minister and chair of the Women and Equalities Select Committee, told Sky News that Mr Johnson, the former MEP and father of the prime minister, inappropriately touched her at the Conservative Party Conference in Blackpool in 2003.

Ms Nokes, who has represented Romsey and Southampton North in the Commons since 2010, was at the time in her 30s and the prospective parliamentary candidate for the constituency ahead of the 2005 general election.

She told Sky News: “I can remember a really prominent man – at the time the Conservative candidate for Teignbridge in Devon – smacking me on the backside about as hard as he could and going, ‘oh, Romsey, you’ve got a lovely seat’.”

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

Sky News approached Mr 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.”

In the wake of the MP making her allegation, New Statesman journalist Ailbhe Rea has said she was “groped” by Stanley Johnson at the 2019 Conservative Party Conference.

Describing her recollection 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, Ms Nokes said: “I would have been in my early 30s, so old enough to call it out.”

On Tuesday, Labour leader Sir Keir Starmer said the allegations of inappropriate touching made against the PM’s father should be “fully investigated”.

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.”

Speaking to Sky News on Wednesday morning, Ms Trevelyan said the Conservative Party has a “robust system” in place for dealing with complaints if Ms Nokes wanted to raise them.

Asked if Mr Johnson will be investigated, she said: “I will leave Caroline to work with the party on that, but we have a robust system in place and I hope very much that she will be able to work that through with the party machine.

“But more widely, this is something that all of us who are women – not only in political life, in all sorts of life – have for far too long had to tolerate.

“The sort of casual sexism, the wandering hand, that is completely unacceptable. And most men are as horrified as you or I are.”

Asked what she would have done in response to the alleged touching, Ms Trevelyan added: “At the time, I would have probably slapped him, which arguably isn’t a better response either, but it would’ve been an instinctive response from me and I think Caroline would’ve shown great personal restraint if she quietly moved away.”

She continued: “Any woman who receives that sort of abusive behaviour from someone absolutely should feel confident that she can stand up, both face them down, but also have the support around her to make sure that the abuser in question does not do that.”

Labour’s shadow Commons leader Thangam Debbonaire said the Conservative Party has a “duty of care” to its MPs and “should be supporting Caroline”.

Ms Debbonaire added that “very rarely is it a one-off” incident when allegations of sexual assault are raised.