A “heavy” presence of officers was reported near a Ministerial corridor, where some Government ministers have their offices, early this afternoon.
The office remains closed and police remain on the scene.
A Met statement said: "At about 11.36am today, Tuesday 13 February, police were informed of a suspicious package that had been delivered to an office within the Palace of Westminster. Police are at the scene and dealing.
"The letter contained a white powder, which is currently being assessed by specialists. The office remains closed at this time, but the rest of the Palace of Westminster is open.
"Detectives from the Met’s Counter Terrorism Command have been informed and are investigating."
A House of Commons spokesperson said: “Today the Metropolitan Police investigated a small package containing white powder on the Parliamentary Estate. The powder was found to be non-harmful.”
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