Andrew Nicholson, one of the most successful riders in eventing history, is in hospital after suffering a neck injury in a fall.
The New Zealander, 54, fell from Cillnabradden Evo at the Festival of British Eventing at Gatcombe Park.
He was taken to hospital in Swindon after Sunday's fall, then transferred to Oxford's John Radcliffe Hospital.
"Andrew has a neck injury and is currently undergoing treatment," a family spokesperson said.
The former world number one has won three Olympic team medals with New Zealand, as well as three World Championship medals.
He is also a five-time winner at Burghley Horse Trials in Lincolnshire.