The 2014 Derby winner Australia has been retired following injury concerns.
Spokesman Kevin Buckley told Channel Four: "He developed a problem in a right hind hoof. The decision was made yesterday to retire him."
Australia was described earlier this year by Aidan O'Brien as the best horse he has trained.
The colt, who will be retired to stud at Coolmore in County Tipperary, had been expected to run on British Champions Day at Ascot next weekend.
Australia was an unlucky third in the 2,000 Guineas at Newmarket in May before winning the English and Irish Derbies and the Juddmonte Stakes at York.
Ridden by the trainer's son Joseph, he made history when winning at Epsom as Aidan O'Brien became the first trainer to win the Derby three years running.
But, he was beaten by The Grey Gatsby on his last run in the Irish Champion Stakes at Leopardstown last month.
His overall record saw five victories from eight starts.
Australia, who earned more than £2m in prize money, was a well-bred son of 2001 Derby winner Galileo and 2004 Oaks victor Ouija Board.
|BBC horse racing correspondent Cornelius Lysaght|
|"Trainer Aidan O'Brien takes a lot of stick for describing his star horses as "the best we've had", especially as the subject of the accolade prior to Australia was Camelot who didn't entirely live up to expectations."Australia was, however, exceptional in winning his two Derbies and the International Stakes at York during an imperious summer campaign, and when beaten on what turned out to be his final run, the jockey was big enough to admit to errors."With his gilt-edged pedigree, as a brilliant son of a union between Classic winners Galileo and Ouija Board, breeders will be beating a path to Australia's door at Coolmore Stud."|
Buckley confirmed the decision to retire him came after consultation between resident farrier at Ballydoyle Jeff Henderson, vet John Halley, and the respective owners.
Henderson accepted the challenge of getting Australia ready for Ascot was over once an infection in the problematic hoof had worsened.
"At the beginning of the week we discovered a bit of soreness in the heel, which made us suspicious of a hoof abscess," Buckley said.
He added that a couple of days later, the horse pulled out lame and, on inspection, the infection had worsened.
Australia would have been one of the star attractions on Champions Day next weekend, but Buckley was adamant the Ballydoyle team could not afford to take any risks at this stage of his career.
"We were all looking forward to going to Ascot, but as you can appreciate, any type of setback like that jeopardises the training regime," he said.
"Hence we had to take that decision he wouldn't be ready for the Champion Stakes."
David O'Loughlin, Coolmore director of sales, added: "Australia nearly won the best Guineas in the modern era.
"He's probably quicker than Galileo."