The dramatic circumstances of Sir Michael Fallon’s sudden resignation as defence secretary last week can be revealed by the Observer.

The cabinet heavyweight’s shock departure on Wednesday followed a phone call from the journalist, Jane Merrick, who informed Downing Street that he had lunged at her and attempted to kiss her on the lips in 2003 after they had lunched together.

I won’t keep my silence: Michael Fallon lunged at me after our lunch | Jane Merrick Read more

The revelation was the tipping point for No 10, which the Observer understands had been compiling a list of alleged incidents involving Fallon since claims against him were first made.

After Fallon’s attempt to kiss her, Merrick, then a 29-year-old junior political reporter at the Daily Mail, said she “shrank away in horror and ran off to my office in the Press Gallery”. Writing in the Observer, she 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.”

A friend of Fallon confirmed the incident took place. “Michael realises this kind of incident was unacceptable which is why he had to resign,” they said.

Merrick said she decided to contact Downing Street after becoming aware of other, more recent allegations against Fallon. She has written about the incident before, choosing not to identify Fallon, but his behaviour after further allegations last week convinced her to name him. She writes that Fallon “has denied some allegations, and minimised others as somehow acceptable because they date from another time. His lack of contrition has made me change my mind.”

On Tuesday, the Sun revealed that Fallon had admitted to repeatedly touching the journalist Julia Hartley-Brewer’s knee at a conference dinner 15 years ago.

Merrick writes: “The impression was being left that this was a one-off incident that could now be laughed off. I knew that by failing to act I was letting down not only my 29-year-old self, but also any other women who may have been subjected to the same behaviour since. More importantly, I would be failing to protect other women in future.”

On Thursday evening, it emerged that the Commons leader, Andrea Leadsom, had informed Downing Street earlier in the week about lewd remarks Fallon had made to her, a claim he denied.

Merrick writes: “Once again, some were dismissing the remarks as trivial, the sort of banter that goes on in workplaces across the country. By continuing to stay silent now, I was still running away from Fallon, just as I ran away from him in 2003. I decided it was time to come forward and name him. I want him to know that, however long ago this incident was, his behaviour was unacceptable.”

Her approach to No 10 triggered a swift reaction. “At 5pm on Wednesday afternoon, I contacted Downing Street to report the 2003 incident to one of the prime minister’s aides,” Merrick writes. “By 7.30pm, Fallon had resigned, admitting his behaviour towards women in the past had ‘fallen short’ of the high standards of the Armed Forces.”

Messages left seeking comment from Fallon went unreturned. However, he told the Times on Saturday: “I’ve already accepted that I have behaved inappropriately in the past.”

Merrick’s account offers a corrective to the suggestion that Fallon was forced out because of his alleged comments to Leadsom. Rather, it seems a pattern of allegedly unacceptable behaviour over the course of many years was the real reason for his spectacular fall. Leadsom told No 10 and the whips’ office about Fallon’s alleged comments early last week. However, the Observer understands she insisted on anonymity and did not want to make a formal complaint. When her allegations leaked, fingers were pointed at the former chief whip, Gavin Williamson, who replaced Fallon. Williamson’s office has denied that he had any role in the leak.

Theresa May can ill afford suggestions of a power struggle between the leader of the house and the defence secretary as the drip-feed of allegations threatens to engulf the government while it is supposed to be focused on Brexit. The toxic claims, which include an allegation that May’s closest ally, Damian Green, behaved inappropriately with a female journalist, a claim he denies, have dismayed many of the Tory old guard who fear what it could do to the party.

Tory MP Sir Roger Gale on Saturday warned of a witch-hunt as accusations of sexual harassment continue to swirl. He said MPs and others were on a “hiding to nothing” as it was difficult to refute claims from years ago. His comments were swiftly rejected by Labour MP Harriet Harman, who told the BBC: “No, it’s not a witch-hunt; it’s long overdue.”

Gale was speaking after the Tory MP Charlie Elphicke said he was unaware of the nature of “serious allegations” against him, which the Tory party says it has passed to police. The chief whip, Julian Smith, has confirmed that Elphicke has been suspended but would not elaborate further. The Dover and Deal MP has denied any wrongdoing and complained that the media had been told of his suspension before he had.

Meanwhile, a string of Labour MPs have had to respond to accusations: Clive Lewis denied a claim that he groped a woman at conference in September; former Foreign Office minister Ivan Lewis rejected claims he had made non-consensual sexual advances towards women; Kelvin Hopkins “categorically” denied allegations of inappropriate conduct made by activist Ava Etemadzadeh.

Shadow chancellor John McDonnell called for party leaders to agree new procedures when they meet later this week. “We have had sexual harassment across all the political parties by the looks of it. So we have got to tackle it,” he said.

And Scottish childcare minister Mark McDonald resigned on Saturday after allegations regarding his private life. The MSP for Aberdeen Donside apologised “unreservedly to anyone I have upset or who might have found my behaviour inappropriate”.

Lord Bew, chairman of the committee on standards in public life, said outside bodies needed to be involved in investigating the allegations. “It is vital there are people outside parliament who can give some reassurance to the public that this is not just another cover-up,” he said.