A spokesman for the South Thanet MP said: “On Friday morning, Mr Mackinlay was made aware of an allegation against a member of his staff.
“Following a request made by the police and parliamentary services he granted full access to his Westminster office. Mr Mackinlay is currently awaiting further information from the police.”
Westminster sources confirmed that the arrested man is Sam Armstrong, Mackinlay’s chief of staff.
Earlier this year Armstrong was accused by the Mail on Sunday of attempting to film Robert Halfon, a senior minister who was having an affair, leaving a club in London. He denied the allegations.
Officers were called to Westminster on Friday after the alleged victim raised the alarm. Police said the man had been bailed until January as detectives investigated the allegations.
A Met spokesman said: “Detectives from the sexual offences, exploitation and child abuse command are investigating an allegation of rape at the Houses of Parliament in the early hours of Friday 14 October.
“A 23-year-old man was arrested that same day on suspicion of rape. He has been bailed pending further inquiries to a date in mid-January 2017.”
A spokesperson for the House of Commons said: “We are aware of an incident on the parliamentary estate, which is being investigated by the Metropolitan police service. Parliament is working closely with the police on their investigation and we cannot comment further while this is ongoing.”