Armed police were seen outside the Palace of Westminster at around midday amid reports that a “suspicious package” was being investigated.
The Met confirmed that they were dealing with reports of an envelope containing white powder.
Specialists are assessing the package and police remain at the scene.
A House of Commons spokesman confirmed an investigation was ongoing but said the building was not in lockdown.
A message was put out on the annunciators which are displayed on TV screens around Parliament, reading: “Incident in the House of Commons being dealt with by the Metropolitan Police, do not be alarmed.”
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.”
Scotland Yard later confirmed that the white powder was not a noxious substance.
In a statement released shortly before 3pm, officers said: “The letter contained a white powder, which has been assessed by specialists and found not to be noxious.
“The office remains closed at this time, but the rest of the Palace of Westminster is open.”