The race meeting at Epsom on Thursday evening has been called off following the terrorist attacks in London.
Officials at the track had earlier indicated that the meeting would go ahead, having been assured there would be adequate ambulance cover.
"The decision was taken in view of the escalating situation in London," said clerk of the course Andrew Cooper.
"We feel it would be inappropriate for us to race and the meeting has therefore been abandoned."
Clerk of the course Andrew Cooper said: "It wasn't a decision that we took lightly and we thought long and hard about it."
Managing director Stephen Wallis said he felt it inappropriate to stage "an entertainment event in the shadow of London".
"Our heartland is suburban London and we felt that tonight, there are people who need to make more important journeys than getting to the races," he said.
British Horseracing Board managing director Greg Nichols said he understood and supported Epsom's
decision, but insisted that as much as possible, the sport should continue unaffected.
"Our first concern is not for racing but for the innocent victims who were going about their daily lives," he said.
All five cards scheduled for Friday are set to go ahead.
At Newmarket, restrictions on the use of mobile phones by jockeys were lifted in order for riders to remain in contact with family and friends.
A public address announcement warned racegoers to be vigilant in looking out for suspect packages, and the atmosphere at the track remained more sombre than usual.
Epsom ticket holders will be offered refunds. Officials are hoping to reschedule a performance by pop singer Ronan Keating, which was due to take place after racing.