Dual Oaks winner Ouija Board will be out of action until at least September and her long-term future is uncertain.
The Ed Dunlop-trained filly, voted Horse of the Year in 2004 after also winning at the Breeders' Cup meeting, was injured at Royal Ascot at York.
She suffered a small stress fracture to her near-fore cannonbone on her return in the Prince of Wales's Stakes.
"It's unlikely she will return to a racecourse before September," said owner Lord Derby.
Ouija Board won both the Epsom Oaks and the Irish equivalent last season before victory in the Breeders' Cup Filly & Mare Turf.
She could be aimed at the American meeting again if mounting a full recovery, but her owner sounded a note of caution.
"We will not hesitate to retire her is there is any doubt about her welfare," he told the Racing Post.