Don Cossack, winner of the 2016 Timico Cheltenham Gold Cup, has been retired after suffering a setback, trainer Gordon Elliott has announced.
Owned by Michael O'Leary's Gigginstown House Stud, Don Cossack held off Djakadam to win the most prestigious prize in National Hunt racing last March.
He subsequently suffered an injury when being prepared for the Punchestown Festival but had been working towards a return to action next month.
However, Elliott reports the 10-year-old to have heat in his leg, with connections opting to retire their star performer.
Elliott told his Betfair blog: ''I am very sad to report that Don Cossack has suffered a setback and will now be retired. We found that he had a bit of heat in his leg yesterday and we've made the decision to call time."
Don Cossack was beaten just twice in 12 starts since October 2014, settling for third in the 2015 Ryanair Chase and falling in the 2015 King George VI Chase when still holding every chance.
Connections had been making bullish noises recently about a return, but they had said from the outset that if there were any forms of complication, he would be retired.
Elliott added: "It's a real sickener for Gigginstown, myself, Bryan Cooper and the whole yard.
"We knew it was never certain we would get him back to the racecourse and even after that, to get him back to his best, but we were hopeful and he was on track for a run at Gowran Park next month.
"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 retire him straight away.
"He's won Grade Ones at Cheltenham, Aintree, Punchestown, Fairyhouse and Down Royal. He was the top-rated horse in Britain and Ireland for the last two seasons running and we'd have loved to see him take on Thistlecrack in the Gold Cup. It would have been a cracking race.
"That's racing as they say and we have to let that go now and find something else to take on Thistlecrack.
"It was one of the highlights of my career when Don Cossack won the Gold Cup for us last year and he retires a champion. As I said already, he owes us absolutely nothing and a peaceful retirement awaits him out in Gigginstown."
O'Leary said: "Gordon left no stone unturned to help Don Cossack return to defend his title but alas it is not to be. At the age of 10 we are all agreed that the best thing for 'The Don' is a well-earned and very well-pampered retirement in Gigginstown."
Cooper partnered Don Cossack to win seven races, including the Gold Cup, and the rider admits that day will be hard to top.
He told 32Red: "He gave me the highest point of my career when we won the Cheltenham Gold Cup last March, and that day will take some beating whatever I now achieve in the saddle.
"I was actually beaten on my first three rides on him and he was a bit disappointing in his novice chase campaign after he won the Drinmore but when I rode him on his return at Punchestown the following season after a summer break, he was a lot stronger and a totally different horse.
"We knew then we had a horse with the ability to reach the top and his only defeats afterwards were a disappointing third in the Ryanair and when he fell when coming there with every chance in last season's King George.
"In my opinion, he was a 10-15lb better horse on good ground, and I think the formbook backs that up, too.
"He may have handled soft ground but that was just his huge natural ability getting him through it, and riding him on decent ground gave you a completely different feel and experience.
"I imagine it will be a long while before I sit on another as good as him. On his day he was outstanding, and at least he bowed out on a high on the racecourse."
Don Cossack's retirement means neither of the last two Gold Cup winners are in contention for the prize this year, with 2015 hero Coneygree ruled out of the race earlier in the week.