One unnamed officer in Speaker’s Court told the Press Association that specialist colleagues were examining a package left in the Palace of Westminster.
They said: “We haven’t been told much, but a package has been left.
“Specialist officers have just arrived to examine it.
“It won’t be anything to worry about.”
A House of Commons spokesman confirmed the investigation was ongoing but the building was not in lockdown, nor were any security alerts displayed on the annunciators around the Palace of Westminster.
No extra officers were visible at the main entrance at Carriage Gates.
A Commons spokesman said: “The police are currently investigating an incident on the Parliamentary Estate.
“While their investigation is ongoing, any questions should be directed to the Metropolitan Police press office.”
A Met Police spokeswoman said: “We are still looking in to it.”