British eventer William Fox-Pitt is in a "stable" condition in a French hospital after suffering a fall on the cross-country course during the World Young Horse Championships.
A statement said the 46-year-old had suffered a "head trauma" after falling from his horse Reinstated, which was unharmed, on Saturday.
"He has undergone tests and will remain sedated under observation," it added.
Fox-Pitt is GB's most successful rider with 20 major championship medals.
His haul includes three Olympic team podium finishes and six European team gold medals for Great Britain.
Fox-Pitt, who is based at Sturminster Newton in Dorset, was competing in the seven-year-old class at Le Lion-d'Angers.