The prominent journalist claimed Fallon "repeatedly put his hand on my knee" during a dinner at Conservative party conference and she "calmly and politely explained to him that, if he did it again, I would "punch him in the face".
Ms Hartley-Brewer said since responded that she "absolutely didn't expect the news".
"A number of allegations have surfaced about MPs in recent days, including some about my previous conduct".More news: Broadcom is reportedly exploring a massive acquisition of $80 billion Qualcomm
He told the BBC that what had been "acceptable 15, 10 years ago is clearly not acceptable now".
Reacting to Fallon's resignation on Twitter, Hartley-Brewer said: "I doubt my knee was the reason".
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.More news: Justin Bieber and Selena Gomez are back together? Here's what sources claim
In a letter accepting Fallon's resignation on Wednesday, May praised his "diligent service".
"I appreciate the characteristically serious manner in which you have considered your position, and the particular example you wish to set to servicemen and women and others", she wrote in a letter.More news: Apple looking beyond Qualcomm for smartphone components
And there were signs of consternation in Conservative ranks, with some backbenchers suggesting that the 41-year-old South Staffordshire MP lacks experience and may have earned his meteoric rise by his staunch loyalty to the Prime Minister.
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