I'll Have Another was dramatically denied a tilt at US racing history at the 11th hour on Friday when injury ruled him out of Saturday's Belmont Stakes and a shot at the US Triple Crown.

The Kentucky Derby and Preakness Stakes hero had been a red-hot favourite to add the Belmont, only for connections to agonisingly discover a tendon problem in a front leg and later retire the colt.

"Pulling him isn't tragic but it's heartbreaking," said trainer Doug O'Neill. "Could he run and compete, yes. Would it be in his best interest? No.
 
"The bottom line is it's not tragic, no one got killed, there are much bigger issues in the world."

I'll Have Another would have been aiming to become only the 12th Triple Crown winner and the first since Affirmed in 1978.

Sympathy came swiftly from trainer Dale Romans, who will saddle Dullahan - likely favourite in the absence of I'll Have Another.

Romans said: "It's a shame, it's sad for the sport," he said "It would have been a great race."

I'll Have Another had gone out to exercise early on Friday morning, galloping at 5.30am. Romans said he had suspected something was amiss when he saw vets attending to the dual Classic-winning colt.

"I had some suspicions when I got to the barn this morning when I saw the vets working on him," he said. "That's never a good sign."

Before a press conference held at 1pm local time, O'Neill, who trains I'll Have Another for Reddam Racing llc, added: "We scanned his left front leg and he's got the start of tendinitis going on in his front leg. He's not 100 per cent and we ain't taking any chances.

"I'm just so bummed for the horse, obviously, and then for the whole team."

Asked whether I'll Have Another had run his last race, O'Neill said: "If I had to wager, I would say yes."

"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."

However, it was reported later that the cheaply bought son of unheralded sire Flower Alley would be retired after a career boasting five wins from seven starts and earnings of $2,629,600.

I'll Have Another became a first Classic winner for O'Neill, for principal owner Paul Reddam, and jockey Mario Gutierrez, who was trying to join the short list of riders who have won a Triple Crown.

Gutierrez, 25, understood the decision, saying: "I'm not disappointed. We have to take care of the horse."

The media spotlight on I'll Have Another had been intense. Even more so because of the unusual decision of New York regulators to insist all Belmont runners were placed in a special detention barn, which served to place even more media scrutiny on O'Neill's chequered career record.

Although Californian-based O'Neill has strenuously denied any wrongdoing, he has a number of medication violations to his name including three of his horses testing positive for high levels of carbon dioxide in their system.

The TCO2 test is designed to uncover cases of 'milkshaking', the illegal practice of giving a horse a blend of bicarbonate of soda, sugar and electrolytes believed to reduce fatigue and enhance performance.

It emerged last month that O'Neill faced another inquiry after one of his horses tested positive to elevated levels of carbon dioxide in August 2010. Although cleared of deliberate malpractice, he received a 45-day ban, to start at the end of the month.

As for the tightened security, he added: "I am kind of looking forward to running him in a situation when you're in a fishbowl and everyone who has thought negative thoughts about me can realize it's not about me, it's about the horse.

"They can watch him lead up to a historical event and, hopefully, watch him kick butt, in a fishbowl. But if I get beat, I'm going to blame the security barn!"