Sir Michael Fallon has apologised for repeatedly placing his hand on a female journalist's knee during a dinner in 2002.
Responding to a story on the front page of The Sun, a spokesperson for the defence secretary said, "he apologised when the incident happened 15 years ago and both Julia and he now considered the matter closed."
The incident involved journalist Julia Hartley-Brewer, who said she does not regard it as "anything other but mildly amusing."
"I have spoken previously about a Cabinet Minister who repeatedly put his hand on my knee during a party conference dinner,” Hartley-Brewer tweeted.
"I calmly and politely explained to him that if he did it again I would 'punch him in the face'.
"He withdrew his hand and that was the end of the matter. I have had no issues since with the man in question and do not regard the incident as anything but mildly amusing."
"This "incident" happened in 2002. No one was remotely upset or distressed by it. My knees remain intact."
A associate of Sir Michael described Hartley-Brewer as “a good friend” of the minister.
"He overstepped the mark when he put his hand on her knee,” the associate said. “She made it clear it was unwelcome and he rightly apologised 15 years ago."
The revelation came amid swirling abuse allegations in Westminster.
Commons Speaker John Bercow met with senior parliamentary figures on the House of Commons Commission to plot reforms to allows victims of sexual abuse to report incidents “without fear” of reprisal.
The latest revelations were published in Monday’s The Daily Telegraph, which reported how a minister's "inappropriate" actions forced two female staff members to move jobs.