The 23-year-old was named in reports as Sam Armstrong, who is chief of staff to Tory MP Craig Mackinlay.
It is alleged that he took a woman to the South Thanet MP’s office after a drinking session in a bar within the Palaces of Westminster in the early hours of Friday morning.
A source told The Sun: “There were a group of Tory advisers and female guests drinking there all night. Some were very drunk indeed.”
The alleged incident is said to have happened after a group were drinking in a taxpayer-subsidised bar on the House of Lords’ terrace overlooking the River Thames.
Sam Armstrong was reportedly arrested on Friday
Mr Mackinlay’s parliamentary office was raided by police on Friday and sealed off as a potential crime scene.
A spokesman for Mr Mackinlay told the Standard: “On Friday morning Mr Mackinlay was made aware of an allegation against a member of staff. Following a request made by the police and Parliamentary officers, he granted full access to his Westminster office.
“Mr Mackinlay is currently awaiting further information from police,” he added.
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.
“A 23-year-old man was arrested that same day on suspicion of rape. He has been bailed pending further inquires to a date in mid-January 2017.”
A House of Commons spokesman 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.”