Northern Ireland Secretary Theresa Villiers was on business in London when security staff spotted the suspicious letter Tuesday. No group claimed responsibility, but police and politicians blamed Irish Republican Army militants who have been behind a spate of similar attacks.
Police evacuated dozens of people from the castle, including the Protestant leader of Northern Ireland's unity government, First Minister Peter Robinson.
Last week Royal Mail staff detected two other letter bombs addressed to the senior police commanders in Belfast and Londonderry. On Monday, a Londonderry office complex housing the British state prosecutor's office was evacuated when another letter bomb was found. None detonated.