Gavin Williamson appointed to replace Michael Fallon as Defence Secretary

The Scarborough-born Williamson, who has enjoyed a speedy assent since being elected as the MP for South Staffordshire in 2010, was appointed less than 24 hours after Sir Michael stood down.

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.

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 department is separately probing whether international trade minister Mark Garnier breached the ministerial code after he reportedly admitted asking his secretary to buy adult toys and made suggestive comments.