"The police are now investigating an incident on the parliamentary estate", a House of Commons spokesperson said, according to Reuters.
While the investigation was originally reported to involve a "suspicious package", a police statement later clarified that it was an envelope containing white powder.
Police said the letter was delivered at around 11.30am (10.30pm AEDT) on Tuesday and is being assessed by specialists.
The rest of Parliament remains open.
Police counterterrorism detectives are investigating the incident. Officers are understood to be examining a "suspicious package".
Cops said the package contained white powder.
A Met Police spokeswoman said: 'We are still looking in to it'.
One anonymous officer in Speaker's Court said that authority associates were analyzing a bundle left in the Palace of Westminster.
There were reports of a heavy police presence in the ministerial corridor of the Commons, where MPs serving in government have their parliamentary offices.
Britain's parliament is now in recess, which means most ministers are not in Westminster.
A House of Commons representative said the examination was continuous however the building was not in lockdown, nor were any security cautions showed on the annunciators around the Palace of Westminster.