For most of its history, Number 10 was accessible to the public. Early security consisted of two police officers: one stood guard outside the door; the other was stationed inside to open it. Since the door had no keyhole, the inside officer depended upon the lone outside officer. During Thatcher's premiership, terrorist threats led to the implementation of a second level of security. Despite the added measure, on 7 February 1991, the Provisional IRA used a van they parked in Whitehall to launch a mortar shell at Number 10. It exploded in the back garden, while Prime Minister John Major was holding a Cabinet meeting. Major moved to Admiralty House while repairs were being completed. The attack led to the addition of guardhouses at the street ends as well as other less visible measures to further improve security of Downing Street.
