Boris Johnson was “shocked” when he was told that a Conservative MP is under police investigation after being accused of rape and sexual assault offences.
The Metropolitan Police said the unnamed man in his 50s, who was bailed pending further inquiries on Wednesday, was also detained on suspicion of indecent assault, abuse of position of trust and misconduct in public office.
Downing Street said the British prime minister was first told of the news about the allegations on Tuesday.
But No 10 would not comment on why the Tory whip had not been removed following the police investigation becoming public knowledge.
Labour has argued the Tories should suspend the whip from the MP in question – a move that would see the accused named.
A spokesman for opposition leader Sir Keir Starmer said: “It’s a matter for the Conservative Party what they do, but our view is that, given the nature of the allegations, then the whip should be suspended and obviously at that point it would become public.”
Tory chief whip Chris Heaton-Harris has urged the MP to stay away from Parliament but has not suspended the whip, meaning the suspect remains a member of the parliamentary party.
UK Commons Speaker Sir Lindsay Hoyle confirmed the suspect had agreed to stay away from Westminster and urged fellow MPs not to reveal his identity.
The prime minister’s press secretary told reporters that Mr Johnson was “shocked to learn of these allegations”.
However, she batted away questions about why MPs had previously been suspended by the Tories during probes but that the same approach was not being taken with the arrested politician.
“I appreciate there will be a lot of questions on this topic but while there is an ongoing investigation I hope you will understand that it won’t be appropriate for me to comment on that specific question,” she said.
“It is a matter for the whips. You will have seen they have commented on this in terms of a decision on the whip being reviewed once the investigation has concluded.”
The spokeswoman went on to suggest that if the MP was to be charged, they could expect to “lose the whip”.
In a statement, Scotland Yard 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 was taken into custody and has since been bailed pending further inquiries to a date in mid-June.”
UK foreign secretary Liz Truss said the latest allegations showed that the “culture” of the House of Commons “needs to change further”.
But she said Parliament’s working environment should not provide an “excuse for people to commit appalling crimes”.
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 past year, around 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.
The prime minister’s press secretary said it was “not acceptable” that a “small minority” of MPs had not lived up to the behaviour expected of them.
Conservative sources declined to identify the individual in the latest case, citing the need to protect the identity of any complainant.
Mr Hoyle said the House authorities took the “safety of our staff and parliamentary community as a whole very seriously and are ensuring any necessary measures are taken in respect of MPs, employees and staff”.
But the Prospect union, which represents hundreds of staff working in the Houses of Parliament, said firmer action is needed.
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.”