The Richard Hannon-trained five-time Group One winner Canford Cliffs has been retired to stud.
The four-year-old suffered a leg injury when second to Frankel in the Sussex Stakes at Glorious Goodwood on July 27.
Hannon said: "He's been a fantastic horse for the yard. He is without doubt the best horse I have had in over 40 years as a trainer.
"It's a terrible shame - it's heartbreaking, but I don't want to see him end up with a bad injury."
He added: "He has a bit of a shadow on the joint running into the pastern and that could turn into something nasty - it could turn into a fracture."
The top miler enjoyed a successful racing career, winning seven of his 11 racecourse starts and finishing either second or third on his other four appearances.
The son of Tagula first burst onto the scene with an electric performance in the Coventry Stakes as a juvenile and victory in the Irish 2,000 Guineas last season began a run of form which saw him win five successive Group One races.
However, the highly-anticipated "Duel on the Downs" with Frankel at Goodwood proved one-sided as Frankel won by five lengths.
Hannon said: "He's won five Group Ones - you can't do any more than that.
"He was a very rare type in that he had such great early speed as a two-year-old but also stretched out to be a superb miler at three and four.
"It's bitterly disappointing for his owners and for everyone here in East Everleigh that he had to be retired due to injury but at least it does explain why he hung so badly at Goodwood.
"Frankel might be a superstar, but we were looking forward to taking him on again in the QEII at Ascot.
"There is no way that Frankel would have given our horse the beating he did had we been right.
"Canford Cliffs did so much for us and we will all miss him, but we have the memories and the videos and, hopefully, we will have some of his sons and daughters back here at Herridge in the not-too-distant future."
Jockey Richard Hughes, who rode the horse, was saddened by Thursday's news but said he had heard "a rumour" the horse might be retired.
"He was a brilliant horse, the best I ever rode from the first day I sat on him and it's nice that he fulfilled his potential," Hughes said.
Canford Cliffs is set to stand at Coolmore Stud in Ireland.