Theresa May is set to appoint a new defence secretary after Sir Michael Fallon became the first ministerial head to roll in the Westminster sleaze scandal.
Sir Michael quit after admitting his behaviour had "fallen below the high standards required" in the role and acknowledging that what might have been acceptable in the past was no longer appropriate.
The resignation leaves the Prime Minister facing a reshuffle and deprives her of one of her most experienced and trusted colleagues.
The move comes as Scottish Conservative leader Ruth Davidson said there needed to be a "house clearing".
Speaking at a political awards ceremony, Ms Davidson said: "Just because we have a woman in Number 10, at the top of the tree in Plaid Cymru, in the Scottish Conservatives, in the SNP, in the DUP, doesn't mean that sexism and misogyny are somehow resigned to the dustbin of history when it comes to politics.
"Nor, when we look at some of the house clearing that ... needs to happen in the next few weeks, months and years ahead, are we going to say that we didn't need some pretty big shovels for the Augean stable.
"The house clearing that is about to happen needs to happen and we can never go back to where we were before."
Sir Michael's shock announcement came after it emerged he had repeatedly put his hand on a journalist's knee at a dinner in 2002.
His name also appeared on an unverified list of sexual misconduct allegations circulating in Westminster.
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 previously said she regarded it "mildly amusing".
She reacted with shock to Sir Michael's announcement, writing on Twitter "bloody hell" before adding "I doubt my knee was the reason" for his resignation.
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.
While Sir Michael has apologised and is not under investigation about the 2002 incident, two of his former ministerial colleagues are the subject of probes.
Cabinet Secretary Sir Jeremy Heywood is looking into claims made against Mrs May's de facto deputy prime minister Damian Green.
The Cabinet Office investigation was launched after activist Kate Maltby, who is three decades younger than the First Secretary of State, told The Times that Mr Green "fleetingly" touched her knee during a meeting in a Waterloo pub in 2015, and a year later sent her a "suggestive" text message after she was pictured wearing a corset in the newspaper.
The department is separately probing whether international trade minister Mark Garnier breached the ministerial code after he reportedly admitted asking his secretary to buy sex toys and calling her "sugar tits".
Ms Davidson said she had discussed the idea of a UK-wide code of conduct for elected officials and political staff with Mrs May.
She told BBC Radio Four's Today programme: "I spoke to the Prime Minister ... at great length about this and what we can do on a UK-wide basis in terms of a code of conduct for all elected officials and those who work for the party too, and we expect both parliaments and parties to move forward from this point.
"The dam has broken on this now, and these male-dominated professions, overwhelmingly male-dominated professions, where the boys' own locker room culture has prevailed, and it's all been a bit bit of a laugh, has got to stop."
Sir Michael's local party said it supports him continuing as MP for Sevenoaks, the seat he has represented since 1997.
Sevenoaks Conservatives chairman Graham Clack told the Press Association: "He's been a fantastic local MP and he will carry on being our fantastic local constituency MP.
"Nothing changes on that."
Education Secretary and Minister for Women and Equalities Justine Greening told BBC Radio Four's Today programme political parties needed to decide what modern standards of behaviour should be.
She said: "There's quite possibly a debate within the public about what those standards should be and there may be different groups of people who think that, actually, standards should be at different levels.
"Some people think that actually we should all expect a bit of this. Perhaps younger people feel that actually this is totally unacceptable. And I think parties are needing to work through what a modern response to this sort of behaviour, and the kind of reports we have seen, should be."
Tory MP Jacob Rees-Mogg told the BBC: "Generally speaking, I think relations between men and women should be on a well mannered, polite basis, and that men have no business going round putting their hands on ladies' knees. I think that is not the way to behave."
Former chief of the general staff Lord Dannatt said Sir Michael had been a "very good" defence secretary.
He told BBC Radio Four's Today programme: "This is a tragedy for defence, and a new secretary of state for defence is needed to make the case for greater spending in this country on defence."