The Metropolitan Police are investigating a suspicious package containing white powder delivered to the Houses of Parliament in Westminster on 13 February.
The office where the letter was sent is currently on lock down as police investigate the letter. However, the rest of the building remains open.
A serous police presence could be seen below the ministerial corridor behind the House of Commons chamber in the historic parliamentary estate.
Sources suggest that only a small part of the building was evacuated because of the package and that the situation is now under control.
A House of Commons spokesman said: "The Police are currently investigating an incident on the Parliamentary Estate.
"While their investigation is ongoing, any questions should be directed to the Metropolitan Police."
Police have promised to issue an official statement shortly. However, the roads surrounding parliament have not been locked down and there are now social media reports of a major incident emerging from the area.
Theresa May and her colleagues are not currently in the building thanks to a parliamentary recess.
On 22 March last year, Kahlid Masood aka Adrian Elms drove a car down the pavement at Westminster Bridge before running into the parliamentary estate and stabbing a policeman.
Masood killed five people including PC Keith Palmer before being shot dead by a marksman responsible for ministers' security.
A pre-hearing inquest at the Old Bailey on 15 January heard of "evidence of anabolic steroids [by Masood] having been taken in the hours or days prior to his death".