Triple Crown aspirant I'll Have Another will not run in the Belmont Stakes on Saturday after reportedly suffering a leg injury.
The son of Flower Alley was a short-price favourite for the race after victories in the Kentucky Derby and the Preakness Stakes.
Trainer Doug O'Neill told the Dan Patrick Show: "I'll Have Another is officially out of the Belmont.
"Pulling him isn't tragic but it's a huge disappointment."
I'll Have Another would have been aiming to become only the 12th American Triple Crown winner of all time, and the first since Affirmed in 1978.
O'Neill said: "We scanned his left front leg and he's got the start of tendinitis going on in his front leg.
"He's not 100% and we ain't taking any chances.
"I'm just so bummed for the horse, obviously, and then for the the whole team."
Asked whether I'll Have Another had run his last race, the trainer said: "If I had to wager, I would say yes."
He added: "I'd love to see him back in my barn in a few months but realistically he'll probably be seeing a lot of beautiful mares and smoking a lot of cigarettes."