Dual Derby winner Australia has been retired to stud, said a Coolmore representative.
In a shock announcement, time has been called on the racing career of the high-class Galileo colt, who had been set to make his swansong on Qipco British Champions Day at Ascot next Saturday.
Kevin Buckley, UK representative for Coolmore, told Channel 4 Racing: "Unfortunately Australia developed a problem in his right-hind hoof during the week.
"Following consultation between the resident farrier at Ballydoyle, our vet, John Halley, and the respective owners, the decision was made yesterday to retire him to Coolmore."
Described by trainer Aidan O'Brien as the best horse he has ever trained, Australia earned over £2million in prize-money during a superb career which yielded five victories from eight starts.
The three-year-old announced himself as a major player in a Group Three at Leopardstown in September 2013, when he accounted for Free Eagle by six lengths.
A narrow third upon his return to action in the 2000 Guineas at Newmarket, he went on to claim victories in the Epsom and Irish Derby, after which he won the Juddmonte International at York.
His tall reputation was, however, slightly tarnished in the Irish Champion Stakes at Leopardstown last month when York runner-up The Grey Gatsby gained his revenge in a dramatic race.
Australia would nonetheless 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.
He said: "We were all looking forward to it (going to Ascot), but, as you can appreciate, any type of setback like that jeopardises the training regime.
"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."
Ballydoyle farrier Jeff Henderson accepted the challenge of getting Australia ready for Ascot was over once an infection in the problematic hoof had worsened.
He said: "At the beginning of the week we discovered a bit of soreness in the heel, which made us suspicious of a hoof abscess.
"We poulticed away at it and the infection came out and there were no soundness issues at all.
"Then a couple of days later, he pulled out lame and, on inspection, we realised the infection had blown out through the bottom of the foot as well as the top.
"In a case like that, it was going to take a couple of weeks for the stability to come back."