NEW YORK (Reuters) - I'll Have Another, the favorite for Saturday's $1 million Belmont Stakes, has been scratched from the race, ending his bid to win the elusive Triple Crown of American thoroughbred racing.

The colt's trainer broke the news on the eve of the highly-anticipated race, saying the three-year-old had suffered a leg injury that could be career-threatening.

"Maybe he just strained himself from training," Doug O'Neill told the Dan Patrick Show on Friday. "Just a freakish thing. I'm bummed we're not participating. Whoever wins will be a special horse."

O'Neill and the horse's owner, J. Paul Reddam, are scheduled to hold a news conference later Friday to discuss the decision to withdraw the horse, whose bid to land the elusive Triple Crown had captured the imagination of millions of Americans.

I'll Have Another had won the Kentucky Derby and Preakness Stakes in his last two starts with devastating finishing bursts and was attempting to become just the 12th horse, and the first since Affirmed in 1978, to complete the treble.

Eleven horses have won the first two legs since 1978 but failed at the last hurdle. I'll Have Another is the first horse since Bold Venture in 1936 to miss the Belmont after winning the Kentucky Derby and the Preakness Stakes.

"It's not tragic, but it's a huge disappointment," said O'Neill. "I have to do what's in the best interest of the horse."

O'Neill said I'll Have Another had injured a tendon during his final workout earlier on Friday. Although the injury was not serious or life threatening, O'Neill said he was not fit enough to compete in the grueling 1-1/2 mile (2,414 meter) race.

"We scanned his left front leg and he's got the start of tendonitis going on in his front leg. He's not 100 percent and we ain't taking any chances," O'Neill said.

What makes the Triple Crown so difficult to achieve is the makeup and timing of the races, each held in different states, over different distances and during a span of five weeks. The winner also invariably faces different opposition each time.

The final race is a lung-bursting test of speed and stamina and the pressure on connections to try and complete the Triple Crown has ended in disaster before.

In 1969, Majestic Prince was withdrawn from the Belmont after suffering a tendon injury but his connections changed their mind and entered the horse even though he was not fit.

He finished a distant second to Arts And Letters and never raced again.

A decade later, Spectacular Bid stepped on a safety pin that became embedded in his hoof and caused an infection that almost killed him. He also raced and was beaten.

In 1999, Charismatic broke two bones in his foreleg while leading in the final stages of the race. Veterinarians saved his life but he never raced again either.

Tendon injuries are common in race horses and most return to the track but O'Neill said it was possible that I'll Have Another, who was bought for $35,000 but has won nearly $2.7 million on the track and was worth double that as a stallion, may not race again.

"If I had to wager ... I would say yes," O'Neill said.