(adds lawmaker comment)
By Elizabeth Piper and William James
LONDON, Nov 2 (Reuters) - British Prime Minister Theresa May was forced to appoint a new defence minister on Thursday after her ally Michael Fallon quit in a sexual harassment scandal that has prompted calls for an end to the "locker room" culture in parliament.
Just hours after Fallon resigned over past behaviour he said had fallen "below the high standards we require of the armed forces", May appointed Gavin Williamson, 41, a rising star in her ruling Conservative Party.
Weakened after losing her parliamentary majority in a June election that many Conservatives blame on her, May had to move swiftly.
The growing harassment scandal threatens to undermine her further if other allegations against Conservatives, including against her deputy Damian Green, force more resignations.
Williamson has risen quickly up the ranks of the Conservatives and May selected him last year as the party's chief whip in parliament - a role in which he is tasked with maintaining voting discipline on legislation.
As chief whip, Williamson was renowned for his toughness and for introducing lawmakers to his pet tarantula Cronus, named after the god of time who swallowed each of his children according to Greek mythology.
But he has no former experience of the armed forces, according to his online profile, and some Conservative lawmakers openly questioned the appointment.
Sarah Wollaston writing on Twitter: "There are times when offered a job that it would be better to advise that another would be more experienced and suited to the role."
Williamson said nothing as he arrived at the Ministry of Defence on Thursday, taking on the role at a difficult time for May, who since June's election is dependent on the support of a small Northern Irish party for a majority in parliament.
Fallon's resignation after he apologised for repeatedly touching a female radio presenter's knee in 2002, was yet another blow.
Described by Conservative sources as a political "Rottweiler", Fallon was a trusted minister able to robustly face down criticism of the government.
Britain's parliament is the latest institution to become embroiled in a sex scandal after abuse allegations against Hollywood producer Harvey Weinstein prompted 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 where powerful people "use positions of power to demand things from others".
"The dam has broken on this now, and these male-dominated professions where the boys-own locker room culture has prevailed and it's all been a bit of a laugh, have got to stop," she told the BBC.
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.
May has ordered investigations into a report that one of her ministers asked a female secretary to buy sex toys and an allegation that her deputy, Green, made an inappropriate sexual advance on a young woman - something he denies. (Additional reporting by Tim Hepher, Kate Holton and Costas Pitas, editing by Stephen Addison)
Our Standards: The Thomson Reuters Trust Principles.