Theresa May is set to select a new Defence Secretary after Sir Michael Fallon became the first scalp of the sexual harassment scandal sweeping Westminster.
The Tory veteran resigned on Wednesday evening, admitting that his behaviour had "fallen below the high standards required" in the role.
The resignation leaves the Prime Minister facing a reshuffle and deprives her of one of her most experienced and trusted colleagues.
Sir Michael's shock announcement came after it emerged he had repeatedly put his hand on a journalist's knee until she threatened to punch him at a dinner in 2002.
His name had also appeared on the unverified list of sexual misconduct allegations which has been circulating in Westminster and on social media.
In his resignation letter to the Prime Minister, Sir Michael said: "A number of allegations have surfaced about MPs in recent days, including some about my previous conduct.
"Many of these have been false but I accept that in the past I have fallen below the high standards that we require of the armed forces that I have the honour to represent."
Asked whether he was worried that there would be further revelations about his behaviour, Sir Michael told the BBC: "The culture has changed over the years, what might have been acceptable 15, 10 years ago is clearly not acceptable now.
"Parliament now has to look at itself and the Prime Minister has made very clear that conduct needs to be improved and we need to protect the staff of Westminster against any particular allegations of harassment."
The 2002 Tory party conference incident involved radio host Julia Hartley-Brewer, who reacted with shock to the resignation.
"If this is over kneegate, him touching my knee 15 years ago and me not having any issue with it today, this is the most insane, absurd and ridiculous resignation of a Cabinet minister ever," she told Sky News.
When allegations of sexual misconduct first began circulating last week, ministers were warned by Downing Street that "serious action" would be taken by Mrs May where necessary.
Sir Michael's resignation will fuel speculation that other ministers could also be forced out as a result of the scandal.
The Prime Minister will hold crisis talks with other Westminster leaders on Monday to discuss plans for tackling sexual abuse and harassment.
She said MPs from all parties are "deeply concerned" about allegations that have emerged in recent days as she invited political counterparts to talks on setting up a new "transparent, independent" grievance procedure.
"We have a duty to ensure that everyone coming here to contribute to public life is treated with respect," she told MPs at Prime Minister's Questions.
Mrs May pulled out of an appearance at the Spectator magazine's Parliamentarian of the Year awards to deal with Sir Michael's resignation.
Shadow home secretary Diane Abbott, who attended the event, said she had "no comment" on Sir Michael but added it was "inevitable" that more allegations would emerge.
"I think the Government's in quite a precarious position," she said as she left the ceremony.
Liberal Democrat leader Sir Vince Cable said he was surprised and saddened by former Cabinet colleague Sir Michael's resignation and speculated that "there must be more" than the "fairly minor" incident with Ms Hartley-Brewer.
"It's unfortunate, he's obviously a very capable guy, we worked together in the coalition, it was comfortable, we came from opposite positions politically but we worked together very effectively.
"So I'm surprised and a bit saddened he's been forced out, none of us understands the underlying reasons."
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Credit: evening standard