"I am very sad to report that Don Cossack has suffered a setback and will now be retired," Elliot wrote in a blog for betfair.com. "We found that he had a bit of heat in his leg on Monday, and we've made the decision to call time.
"He's a horse of a lifetime and he owes us nothing. I said all season that if he had any sort of setback at all we would not abuse him and we'd retire him straight away."
Nine-year-old Don Cossack, owned by Ryanair chief executive Michael O'Leary's Gigginstown House Stud and trained in Ireland, won the Gold Cup in March as a 9-4 favourite with Bryan Cooper in the saddle.
The horse suffered the injury in April.
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(Reporting by Alan Baldwin; Editing by Hugh Lawson)
