Stanley Johnson, the former MEP and father of the prime minister, has been accused of inappropriately touching a senior Conservative MP.
Caroline Nokes, a former minister and chair of the Women and Equalities Select Committee, said the incident took place at the Conservative Party Conference in Blackpool in 2003, when she was in her early 30s.
Ms Nokes, who has represented Romsey and Southampton North in the Commons since 2010, was at the time 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. I have no response. Hey ho. Good luck and thanks."
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Describing her recollection, Ms Nokes said: "I would have been in my early 30s, so old enough, old enough to call it out."
She added: "I now regard it as a duty, an absolute duty, to call out wherever you see it. Be the noisy, aggravating, aggressive woman in the room because if I'm not prepared to do that, then my daughter won't be prepared to do that… you do get to a point where you go 'up with this, I will not put'."
The comments 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.
The discussion, which also featured Labour MPs Jess Philips and Rosena Allin-Khan, and Conservative MP Fay Jones, was intended to examine their views as legislators about what should be done.
But during the conversation each of the politicians also described their own experiences.
In addition to her account in relation to Mr Johnson, Ms Nokes said she was groped as an 11-year-old schoolgirl by a group of men.
Ms Philips, the shadow minister for domestic violence and safeguarding, and a former Labour leadership contender, said she was assaulted by a former boss at a party.
Dr Rosena Allin-Khan, shadow minister for mental health and A&E doctor, said she was harassed by a senior medic when she was a junior doctor.
Fay Jones, who was elected as the Conservative MP for Brecon and Radnorshire in 2019, described how she was flashed by a man when she was 17 years old.
All four women told Sky News they believed public sexual harassment should be made a specific crime in upcoming legislation.
Sky News understands a move to create the offence within the Police, Crime, Sentencing and Courts Bill, which is currently going through parliament, is supported by Home Secretary Priti Patel, but is yet to have approval from Downing Street.
You can read full analysis and watch an extended report on Sky News from Tuesday morning