The evacuation followed a telephone warning to the BBC's Belfast newsroom when a man claiming to represent the Continuity IRA said a car bomb had been left at the course.
It was the second time in recent years that a bomb scare has ended a big race meeting. In 1997 the Grand National at Aintree was abandoned after a similar security hoax.
At Kempton Park, four of the day's six races had been run by the time of the evacuation. Sue Ellen, managing director of United Racecourses, said officials decided to delay the evacuation for almost an hour after the warning was passed from the police and until the main race of the day, starting at 2.20pm, was over. "We received a message from the police just before the King George VI steeplechase. We ran the big race which was what everyone had come for and we felt this was the thing to do." The caller to the BBC in Belfast at 1.30pm said he was from the Continuity IRA, a splinter group opposed to the Ulster peace agreement. He said a device would go off at 5pm. He gave a codeword, but it was not one known to the security forces.
Ms Ellen said: "We had to decide whether we thought this was real or whether it was a hoax but if there was a device it was not due to go off for a period of time. If we had been told it would be 10 or 15 minutes before the device was due to go off we would have come to a different outcome. Weighing up all the circumstances we felt we had time to get people away in an orderly fashion with their cars."
The Metropolitan Police was told of the call at 1.48pm and a spokeswoman said: "We conducted a number of inquiries and as a result the management took the decision to close the course."
At 2.53pm, officials asked people to leave the course. An eyewitness said: "The first few people ran out, some with children, obviously quite anxious to get their children away from the ground. Others seemed quite calm - in fact many people were still clutching their glasses and bottles of champagne."
The Independent's racing correspondent, Richard Ed-mondson, who was at Kempton, said: "There was no panic. The racing fraternity are getting used to these bomb threats."
More than two hours after the evacuation alert, motorists were still trying to leave the car park. No device was found.
Last month, intelligence sources said they believed maverick republican paramilitaries were plotting Christmas campaigns on both sides of the Irish Sea to wreck the peace process. Irish security services said the dissidents included the Continuity IRA, the only paramilitary organisation not to have declared a ceasefire, rebel members of the Real IRA, the group behind the bomb attack last year on Omagh, Co Tyrone, and the Irish National Liberation Army (INLA).
The RUC Chief Constable, Sir Ronnie Flanagan, has repeatedly warned that dissident republicans continue to pose a threat in Northern Ireland and on the British mainland.
Republicans in general remember the postponement of the Grand National as a stratagem that generated huge publicity for the IRA for a relatively small outlay. On that occasion no bomb was planted at Aintree, but the hoax came soon after incidents when bombs were used to disrupt motorway traffic.
At Kempton, the highlight of the day, the pounds 110,000 Pertemps King George VI Chase, was won by the 5-2 favourite See More Business ridden by Mick Fitzgerald. It was the second time in three years that the horse had won this race.Today's meeting at Kempton Park will be subject to a 7.45am inspection "to examine contingencies".
Police dilemma, page 2
Race report, page 16