Police were investigating an incident in the British parliament on Tuesday, the lower house of the legislature said, and media reports said officers were checking a suspect package.
The Met confirmed they were alerted to a suspicious package delivered to the Palace of Westminster - a day after Donald Trump's daughter-in-law was taken to hospital when she opened a letter containing an unknown substance.
A Met spokesman said: "The letter contained a white powder which is now being assessed by specialists".
One office remained closed as officers carried out investigations, but the rest of the building was open, police said.
Scotland Yard said its Counter Terrorism Command is investigating.
Cops said the package contained white powder.
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Ian Gregor, a spokesman for the Federal Aviation Administration, said the agency is investigating the incident, Reuters reports. In 2010, a Qantas A380 had a near-disaster when huge chunks of one of its engine casings blew off four minutes after take-off.
Staff were forced to evacuate as police swooped to investigate.
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 "heavy" presence of officers was reported on a ministerial corridor, which is below the House of Commons chamber, and where some government ministers have their offices.
A section of Parliament has been placed in lockdown after a "suspicious package" was found in a corridor. Detectives examined the letter, which was later found to be "non-harmful".
Witnesses had earlier said police had told them they were dealing with a "suspicious package", which was confirmed by an unnamed officer.
Parliament is now in recess, with MPs on a break until 20 February.