Three-time Coronation Cup winner St Nicholas Abbey has died after losing a battle against serious illness.
The seven-year-old, who was trained by Aidan O'Brien, won six Group One races but suffered a leg fracture last summer.
His owners Coolmore Stud in Ireland helped the horse when he suffered from life-threatening laminitis.
But Coolmore said St Nicholas Abbey had to be put down on Tuesday on "humane grounds".
An ante-post favourite for the 2010 Derby at Epsom, he missed the race through injury but went on to win the Coronation Cup at the course a record three times from 2011 to 2013.
In a statement, Tipperary-based Coolmore said: "This is extremely unfortunate as St Nicholas Abbey had been in terrific form, the laminitis was resolving very well and the fracture had healed better than expected.
"Coolmore would like to thank the surgeons, the international experts and all the staff at Fethard Equine Hospital who gave him such excellent care 24/7.
"We would also like to thank the multitude of well-wishers for all the cards and messages of support for St Nicholas Abbey.
"He will be buried in the graveyard here at Coolmore."
O'Brien's son Joseph guided the colt to victory four times, including historic victory in the 2011 Breeders' Cup Turf at Churchill Downs in the United States.
Joseph, then aged 18 years and five months, was the youngest jockey to win a Breeders' Cup race.
"It was my first big win on the world stage and my first big win outside Ireland," he told the Racing Post.
"What happened to him was very sad. Everyone would have liked to see him stand at stud."