Some of Britain's politicians are being told they've let down the very people they're supposed to serve.

A clear message from Scottish Tory leader Ruth Davidson, after dozens of MPs are accused of sexual harassment and assault.

Defence Secretary Michael Fallon is the first to have resigned over the scandal.

He says his behaviour may have been acceptable 10 years ago, but isn't acceptable now.

But Ruth Davidson told the BBC it was never acceptable.

"When you put people in a position where they feel they can't speak out because they're on a ladder, and the person that is exerting power and influence on them is a few rungs up that ladder then we've failed."

Prime Minister Theresa May has named Gavin Williamson as the replacement for Fallon, the first resignation in a sexual harassment scandal that has prompted calls for change to parliament's "locker room" culture.

The loss of Fallon, described by Conservative sources as a political "Rottweiler", leaves May with a hole in her cabinet, which is already at odds on everything from Britain's departure from the European Union to the government's austerity agenda.

In his resignation letter to May, Fallon, who had apologised earlier this week for repeatedly touching a radio presenter's knee in 2002, said there had been many allegations about MPs, 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," he said, offering no detail on the nature of any other allegations.

May said she appreciated "the characteristically serious manner" in which Fallon had considered his position and "the particular example you wish to set to servicemen and women and others".

May was expected to announce a new defence minister on Thursday.

Sexual abuse allegations against Hollywood producer Harvey Weinstein have prompted hundreds of thousands of women and men to share stories about improper behaviour.

Ruth Davidson, leader of the Conservatives in Scotland, said it was time to break with a culture in politics when powerful people "use positions of power to demand things from others".

Allegations of sexual abuse have ranged from a charge of rape by an activist in the opposition Labour Party by a senior party member, to unconfirmed details of serial "sex pests" on a list reportedly drafted by aides and researchers in parliament.

The prime minister said on Wednesday that action would be taken when there were allegations and evidence of sexual misconduct.