The Chatsworth Horse Trials in Derbyshire have been cancelled.
Race organisers say that the recent wet weather has forced them to abandon the event.
"With huge regret, Chatsworth has been abandoned," organisers said.
"There has been 22ml of rain in six hours, and the ground is totally waterlogged.
"Even with good weather from now on, the footings would be unsafe. The stables are flooded."
Chatsworth's CIC three-star World Cup competition had been due to begin tomorrow with dressage.
Potential Olympic team members Zara Phillips, Lucy Wiegersma, Tina Cook, Oliver Townsend and Sarah Cohen were due to compete.
The final team for London 2012 will be announced on May 24.