British defence minister Sir Michael Fallon has resigned after a newspaper article said he touched radio presenter Julia Hartley-Brewer's knee in 2002.

"A number of allegations have surfaced about MPs in recent days, including some about my previous conduct," Sir Michael said in a letter to Prime Minister Theresa May.

"Many of these have been false but I accept that in the past I have fallen below the high standard we require of the Armed Forces that I have the honour to represent."

Michael Fallon's resignation letter.

The BBC reported on Monday that Sir Michael's spokesman said he had apologised for the incident at the time.

Ms Hartley-Brewer tweeted on Monday that a Cabinet minister had repeatedly put his hand on her knee during a party conference dinner.

"I calmly and politely explained to him that, if he did it again, I would 'punch him in the face'," she said.

"He withdrew his hand and that was the end of the matter."

Ms Hartley-Brewer added that she does not consider herself a victim and "I don't wish to take part in what I believe has now become a Westminster witch hunt".

She later tweeted that "no-one was remotely upset or distressed" by the 2002 incident.

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After sexual abuse allegations against Hollywood film producer Harvey Weinstein prompted hundreds of thousands of women and men to share stories about inappropriate behaviour, Britain's parliament — a bastion of tradition — has been no exception.

Earlier, Ms May said action would be taken when there were allegations and evidence of sexual misconduct.

"I am very clear that we will take action against those where there are allegations that we see, and the evidence is there, that there has been misconduct," she told lawmakers.

Reuters