SANS Frontieres, winner of the Irish Field St Leger in 2010, has been retired due to the recurrence of a tendon injury.
The six-year-old entire was being prepared for a return to the track for the first time since his Classic triumph at the Curragh when he sustained his latest setback.
It is hoped a career at stud awaits the son of Galileo.
Owned by Robert Ogden and trained by Jeremy Noseda, he was restricted to just nine starts.
His most important successes came in his last three outings, taking in the Princess of Wales's Stakes at Newmarket and the Geoffrey Freer Stakes at Newbury before his finest hour in Ireland, almost exactly two years ago.
"We have unfortunately had to retire Sans Frontieres," said the owner's racing manager, Barry Simpson.
"We got him very near to a run and he has been in tremendous form recently and working very well, but sadly he has damaged his tendon again.
"The good thing is that he is still a colt. He is a triple Group winner and hopefully he will have some future as a potential stallion.
"He is wonderfully bred by Galileo out of a Shirley Heights mare (Llia), so he may well appeal to one or two of the studs.
"He was 120 rated and never fulfilled his potential.
"It was only when he won three consecutive Group races that he started to show his ability and sadly we have not been able to capitalise on that.
"Nevertheless, he has been a terrific horse and a great flag bearer for Sir Robert.
"He was very lightly raced, but he had tendon trouble following the Irish St Leger before he was due to go to America for the Breeders' Cup.
"We have brought him back twice and got near to running both times, but sadly he damaged his tendon again and therefore we have no other alternative but to retire him.
"He has got a future ahead of him, hopefully."