A female journalist reported Michael Fallon to Downing Street for attempting to lunge at and kiss her, it has emerged.
Jane Merrick alleged that the former defence secretary made tried to kiss her on the lips after a lunch in 2003.
The Observer reported that a call to No 10 from Ms Merrick helped pave the way to the Cabinet minister's sudden departure from his role this week.
She claimed she decided to formally complain after seeing similar allegations come to light, the newspaper said.
Writing about the encounter, which took place when she was a 29-year-old junior political reporter at the Daily Mail, Ms Merrick said: "I felt humiliated, ashamed.
"Was I even guilty that maybe I had led him on in some way by drinking with him? After years of having a drink with so many other MPs who have not acted inappropriately towards me, I now know I was not."
The journalist had previously written about the alleged incident, but chose not to name Michael.
Her allegations come as the sexual harassment scandal engulfing Westminster shows little sign of abating.
Labour's Harriet Harman hit back at claims that a witch hunt against politicians was under way.
The former deputy party leader told the BBC: "There are a lot of men saying this has been totally blown out of all proportion, this a witch hunt. No, it's not a witch hunt, it's long overdue."
But Tory backbencher Roger Gale said MPs and other prominent figures were on a "hiding to nothing" as it was difficult to refute claims about alleged incidents years ago.
Giving a hypothetical example of a woman claiming a man kissed her in a lift five years ago, Roger told BBC Radio 4's Today programme: "How does a Member of Parliament refute that? It's a witch hunt."
Shadow foreign secretary Emily Thornberry said she was ashamed at some of the "disgusting" allegations of sexual abuse in the Labour Party.
Ms Thornberry, who said she had experienced inappropriate behaviour, told the BBC: "Some of the things that I have heard in the last week have been so disgusting and I am ashamed that this could happen in the Labour Party."
Shadow chancellor John McDonnell called for party leaders to agree new independent procedures when they meet next week to discuss how to deal with sexual harassment claims.
He told the BBC: "We have had one of our members come forward and said that she was raped. That is just unacceptable.
"We have had sexual harassment across all the political parties by the looks of it. So, we have got to tackle it."
Chairman of the Committee on Standards in Public Life, Lord Bew, said outside bodies need to be involved in harassment investigations to avoid claims of a cover-up.
He told the BBC: "The parties need to get a grip of this issue of accountability. It's vital that these things are not seen to be done in-house.
"It is vital that there are people outside Parliament in cases of harassment, and so on, who are there, who can give some reassurance to the public that this is not just another cover-up."
As Westminster was hit by fresh claims of inappropriate behaviour, three MPs denied wrongdoing.
Norwich South MP Clive Lewis denied a claim that he groped a woman at the Labour conference in September after it emerged that the party was investigating a formal complaint against him.
Former Foreign Office minister Ivan Lewis denied he had made non-consensual sexual advances towards women.
The Bury South MP made the remarks after BuzzFeed News reported that a woman alleged he touched her leg and invited her to his house when she was 19 and at a Labour Party event in 2010.
Mr Lewis said he had "never made non-consensual sexual comments or sexual advances to women".
Suspended Labour MP Kelvin Hopkins said he categorically denied allegations of inappropriate conduct made by activist Ava Etemadzadeh, which the party is investigating.