A Tory MP remains in custody after being arrested on suspicion of rape and sexual assault offences spanning seven years.
Scotland Yard said the unnamed man in his 50s was also detained on suspicion of indecent assault, abuse of position of trust and misconduct in public office.
Tory chief whip Chris Heaton-Harris urged the MP to stay away from Parliament but has not suspended the whip, meaning the suspect remains a member of the parliamentary party.
The Metropolitan Police said officers received a report in January 2020 relating to alleged sexual offences committed in London between 2002 and 2009.
It was understood the Conservatives will not make a decision on whether the suspect has the whip suspended until after the police investigation is concluded.
The Prospect union, which represents hundreds of staff working in the Houses of Parliament, said firmer action is needed.
Deputy general secretary Garry Graham said: “What will it take for Parliament to finally take its responsibility to its staff and visitors seriously and suspend access to the estate for parliamentarians under investigation for sexual offences?
“Voluntary agreements to stay away do not work, as demonstrated by Imran Ahmad Khan’s attendance at Westminster whilst investigations were ongoing, despite agreeing to stay away.
“Parliament has the same responsibilities towards its staff as any other workplace and it must live up to them.”
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The arrest comes amid renewed scrutiny of sleaze in Westminster following a string of cases.
Former Conservative MP Imran Ahmad Khan quit the Commons earlier this month after being convicted of sexually assaulting a boy.
David Warburton had the Conservative whip withdrawn after allegations of sexual harassment and cocaine use emerged.
Over the last year, about 15 MPs are suspected to have been investigated by Parliament’s Independent Complaints and Grievance Scheme (ICGS) watchdog that examines allegations of bullying, harassment or sexual misconduct.
Neil Parish stood down as the Conservative MP for Tiverton and Honiton earlier this month after admitting watching pornography in the Commons.
The latest arrest also follows a Conservative MP being arrested on suspicion of rape in 2020. He was never identified and police took no further action after an investigation.
A Scotland Yard statement said: “In January 2020, the Met received a report relating to alleged sexual offences having been committed between 2002 and 2009.
“The offences are alleged to have occurred in London.
“An investigation is ongoing, led by officers from central specialist crime.
“A man, aged in his 50s, was arrested on suspicion of indecent assault, sexual assault, rape, abuse of position of trust and misconduct in public office.
“He remains in custody.”
Conservative sources declined to identify the individual, citing the need to protect the identity of any complainant.
A statement from the Tory whips’ office said: “The chief whip has asked that the MP concerned does not attend the Parliamentary estate while an investigation is ongoing.
“Until the conclusion of the investigation we will not be commenting further.”