The attack had been years in planning. Throughout the Troubles, the Provisional IRA used homemade mortars (known in the trade as "barrack busters") against police and army targets in Northern Ireland, most devastatingly in the 1985 Newry mortar attack, which killed nine officers of the Royal Ulster Constabulary. The Army Council had thought hard about Downing Street as a target. They had considered a car bomb, parked on a nearby street and detonated by remote control as the Prime Minister's official car drove past, but the Council rejected the plan because the likely civilian casualties would have been politically counterproductive. Margaret Thatcher had been the top name on the list, surviving the Brighton hotel bombing of 1984 by minutes. By February 1991 Thatcher was out and John Major was in. The plan had changed shape; the target had not.

The mortar tubes fired one after another, each shell weighing 140 pounds. From a launching position 250 yards across Whitehall, with no direct line of sight to the back garden of Number 10, the IRA volunteer had estimated the angle by map. One former British soldier later described the aim as "remarkably good," the kind of accuracy many trained army crews would have been pleased to achieve. Two of the three shells overshot the target and landed on a grass strip nearby, where they failed to explode properly. The third detonated in the back garden of Number 10, 30 yards from the Cabinet Office window where Major and his ministers were sitting. The blast scorched the back wall, smashed windows, and dug a crater several feet deep in the lawn. The blast wave hit the bomb-proof netting on the windows and dispersed.

Inside the Cabinet Office, every person in the room ducked under the table. The reinforced windows held. Nobody around the cabinet table was hurt. Four other people received minor injuries, including two Metropolitan Police officers stationed nearby. When the explosion's aftershock had died away and the room fell quiet, Major straightened his jacket and said, with the kind of dry understatement that defined his style of leadership, "I think we had better start again, somewhere else." Less than ten minutes later the cabinet reconvened in the Cabinet Office Briefing Room, the secure space deeper inside the Whitehall complex now usually known by its acronym, COBRA. The Gulf War strategy meeting continued. Decisions about the bombing campaign over Baghdad were taken as if nothing had happened, which was both performance and substance: the British government had to be seen to continue functioning.

If the angle of fire had been five or ten degrees different, security analysts later said, the third shell would not have landed in the garden but inside the Cabinet Office itself, and the war cabinet would have been killed. The IRA had come painfully close to assassinating a British Prime Minister, his Foreign Secretary, his Defence Secretary, his Chancellor, and the head of the armed forces, all on the same morning. They missed by about fifteen feet and a few degrees of elevation. The attack was later celebrated in Irish rebel song. The band The Irish Brigade released a track called "Downing Street," set to the tune of "On the Street Where You Live" from My Fair Lady, with lyrics including the line "while you hold Ireland, it's not safe down the street where you live." The melody and the threat sit uneasily together; that was the point.

John Major temporarily moved to Admiralty House while the bomb damage to Number 10's back wall and garden was repaired. The repairs were comparatively minor, mostly cosmetic. The political consequences were not. Within months, the Downing Street security regime had been transformed: permanent guardhouses appeared at both ends of the street, and the ironwork gates that now separate Downing Street from the public on Whitehall were installed, marking the end of the long era when ordinary people could walk past the front door of Number 10 on their way to Trafalgar Square. The IRA campaign continued for another seven years before the Good Friday Agreement of 1998 finally created a political framework that could end it. The cabinet that ducked under the table that morning kept governing. Major went on to win the 1992 general election. The lawn at the back of Number 10 was reseeded, and a 1991 photograph of the crater is one of the few visible reminders that an army once tried to kill a British government by firing artillery at a kitchen garden.