Cheltenham Gold Cup prospect Farmer Jack has died while training on the gallops on Wednesday morning.
The Philip Hobbs-trained nine-year-old was added late to Friday's blue riband event at a cost of £17,500.
Farmer Jack landed the Aon Chase and the Racing Post Chase on his last two starts and was a general 12-1 chance.
"It happened about 0830 - he was having an ordinary canter and just dropped dead," said Hobbs. "It was a pulmonary haemorrhage - to do with his lungs."
Somerset-based Hobbs added: "It's very sad. The horse had been in great form.
"There's no logic to it. It's very rare when a horse is healthy and right in itself."
Farmer Jack's death follows Tuesday's withdrawal of Kingscliff and Rule Supreme, while three-time champion Best Mate scratched last week.
Hobbs, who won the Champion Hurdle with Rooster Booster in 2003, trains Gold Medallist, the favourite for Wednesday's opening race, the Royal & SunAlliance Novices' Hurdle.