A Metropolitan Police spokesman said: "At approximately 1136hrs on Tuesday, 13 February, police were informed of a suspicious package that had been delivered to an office within the Palace of Westminster".
The Metropolitan Police said specialists are assessing the powder.
One office remained closed as officers carried out investigations, but the rest of the building was open, police said.
Scotland Yard confirmed they had been called to the scene just after 11.30am and that its Counter Terrorism Command was investigating.
It was later revealed that a white powder was found inside the suspect package.
A "heavy" presence of officers was reported near a ministerial corridor, where some Government ministers have their offices, shortly before 1pm.
A House of Commons spokesperson confirmed an investigation was ongoing but the building was not in lockdown, nor were any security alerts displayed on the annunciators around the Palace of Westminster.
Vanessa Trump, wife of Donald Trump Jnr, was taken to hospital after opening the envelope addressed to her husband and complaining of difficulty breathing, but the powder was likewise found to be non-harmful. Officers are understood to be examining a "suspicious package".
Parliament is now in recess, with MPs on a break until 20 February.