A spokesman for the Commons said of the investigation: "The Police are now investigating an incident on the Parliamentary Estate".
A suspicious package delivered to an office in the Houses of Parliament contained "non-harmful" white powder, British police has said.
Police said the letter was delivered at around 11.30am (10.30pm AEDT) on Tuesday and is being assessed by specialists.
'The letter contained a white powder which is now being assessed by specialists.More news: Batman Ninja English trailer and release date unveiled
One office remained closed as officers carried out investigations, but the rest of the building was open, police said.
The incident comes just a day after an envelope containing a white powder was sent to the son of US President Donald Trump.
The Home Secretary is now in America, but her staff said they were "thrown into a panic" by security after the incident this morning.More news: 'Thirteen Reasons Why' Author Expelled From Writers Org For Sex Harassment
Officers later affirmed that the substance routed to the home secretary was innocuous.
No extra officers were visible at the main entrance at Carriage Gates.
Relatively few MPs and peers are in Westminster as both houses are in recess, and the package is thought to have been opened by a member of staff.
An unnamed police officer in Speaker's Court in Parliament said a suspicious package has been found.More news: Winter Olympics: Japanese snowboarder Yuto Totsuka in scary crash during halfpipe final