Counter-terror police were called to the Parliamentary Estate this evening after a "suspicious bundle" containing white powder was sent to Amber Rudd.
Police said the letter was delivered at around 11.30am (10.30pm AEDT) on Tuesday and is being assessed by specialists.
"The letter contained a white powder is now being assessed by specialists".
The rest of Parliament remains open.
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Cops said the package contained white powder.
"While their investigation is ongoing, any questions should be directed to the Metropolitan Police press office".
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'.
The complex was not evacuated or placed in a lockdown.
Officers later affirmed that the substance routed to the home secretary was innocuous. Specialist officers have just arrived to examine it.
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An unnamed police officer in Speaker's Court in Parliament said a suspicious package has been found.
He said: "We haven't been told much, but a package has been left".
Most MPs are not in Westminster this week as Parliament is in recess for school half-term.
Police increased security at the site after an Islamic State-inspired attacker drove a auto into pedestrians on Westminster Bridge a year ago, killing four of them, and then stabbed a police officer to death nearby.
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Idsinga said last week that police only found human remains in the planters they confiscated from the Mallory Crescent property. But she said police will primarily now focus on looking through planters found at that property and elsewhere around the city.