Valyra, the unbeaten French Oaks winner, has been put down after suffering a serious injury while being prepared for the Qatar Prix de l'Arc de Triomphe.
The filly, owned by the Aga Khan and trained by Jean Claude-Rouget, fractured her femur while exercising on the beach at Deauville on Wednesday morning.
"She fractured her femur this morning while she was in training," said the owner's racing manager Georges Rimaud.
"It was on the beach at Deauville where they go for a little walk in the early morning. It could have happened anywhere.
"It was a bad fracture of the femur. We looked at all solutions to try to come up with a surgical solution but unfortunately we had to let her go."
Valyra made her debut in April and won all her three starts, culminating in the Prix de Diane at Chantilly in June.
She was prominent in the ante-post market at around 12-1 for the Arc at Longchamp on 7 October.