The racing career of Harbinger, officially rated as the best horse in the world after his eleven length King George victory, is to be decided after the champion colt fractured his near-fore cannon bone during a routine gallop on Saturday morning.
The four-year-old Sir Michael Stoute charge was taken for X-rays straight away which confirmed the fracture.
Harry Herbert, racing manager for owners Highclere Thoroughbred Racing, said, "This is a bitter blow for all 12 of Harbinger's (syndicate) owners and indeed all who are closely connected with the horse, as well as his many fans to whom he has become an icon following his amazing King George victory.
"A decision as to Harbinger's future will be taken imminently," he stated on the owners website.
His next event was to be the Juddmonte International at York later in October, while after that event the Prix de l'Arc de Triomphe was on the cards.
Accumulating over Â£770,000 in prize-money so far, the son of Dansili has had victories in his last four events, including the Hardwicke Stakes at Royal Ascot.
John Warren, co-owner of Highclere Stud, commented, "It's a devastating blow for the British racing public."
"It's very sad news, for the horse and for everybody. He was the hot favourite for the Arc, which everybody was waiting for," said Olivier Peslier, who was aboard the horse at Ascot.
"He's a fantastic horse and, when I received the news, I was very sad because when you have a champion like that, it's a big pity to finish the career like this."
He commented, "He looked like the best. I have ridden Goldikova, Peintre Celebre, but he was a wonderful horse and ranks very high.
"He's unlucky and, for the future, I don't know what will happen. But his safety is more important."