Sam Armstrong, chief of staff to South Thanet MP Craig Mackinlay, was arrested last week.
It is thought the alleged rape took place inside the MP’s office after drinks on the House of Lords’ terrace on Thursday night.
The 23-year-old has been relesaed on bail while inquiries into the incident continue.
A spokesperson for Mr Mackinlay told The Independent: “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.”
He added: “Mr Mackinlay is currently awaiting further information from the police.”
Mr Macinlay’s office was searched by police on Friday and sealed off as a potential crime scene.
A Metropolitan Police 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.