Police closed off a section of Parliament after an package containing white powder was sent to Amber Rudd, the Home Secretary.
Sources told The Independent the envelope included a rambling letter on the issue of radicalisation and made reference to powder.
Specialist officers were called to the Palace of Westminster to assess the substance and later confirmed that it was harmless.
The corridor, which is below the House of Commons chamber, is where a number of government ministers have their offices. It also includes conference rooms for ministers' meetings. Parliament is in recess this week, so most MPs are not in Westminster.
A House of Commons spokesperson said: "Today the Metropolitan Police investigated a small package containing white powder on the Parliamentary Estate. The powder was found to be non-harmful.”
A spokesperson for the Metropolitan Police said officers were called at 11.36am to reports of a "suspicious package delivered to an office within the Palace of Westminster".
"The letter contained a white powder which is currently being assessed by specialists."
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