The spokesperson said: "The Police are now investigating an incident on the Parliamentary Estate".
They added: "The letter contained a white powder which is now being assessed by specialists". The workplace stays shut as of now, however whatever remains of the Palace of Westminster is open'.
Other buildings were not on lockdown and visitor tours were allowed to continue while officers dealt with the situation.
Officers later affirmed that the substance routed to the home secretary was innocuous.
The package is now being investigated by specialists from the Metropolitan Police.
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.
A police vehicle leaves the Palace of Westminster during the investigation into a suspect package.
The Palace of Westminster was not locked down and the main Carriage Gate entrance - the scene of the attack on Westminster last March - was open as normal. "Do not be alarmed". Officers are understood to be examining a "suspicious package".
They said: "We haven't been told much, but a package has been left. It won't be anything to worry about".
"The Police are now investigating an incident on the Parliamentary Estate", the spokesperson confirmed.
An unnamed police officer in Speaker's Court in Parliament said a suspicious package has been found.
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