I'll Have Another will not be the first Triple Crown winner in 34 years -- the thoroughbred has been scratched from tomorrow's Belmont Stakes ... and may never run again.

The horse's trainer Doug O'Neill just dropped the bombshell on "The Dan Patrick Show" ... confirming the rumors that I'll Have Another was indeed injured. He was not specific, but said there was a "little problem" with the left hind leg.

O'Neill said his horse seemed fine "to the eye" -- but clearly, something changed this morning ... just one day before I'll Have Another could have become just the 12th Triple Crown winner in history.

I'll Have Another had already won the Kentucky Derby and the Preakness -- but O'Neill says the horse will not race again for at least 3 months, and could possibly be done for good.

A news conference is scheduled this afternoon in New York.


10:20 AM PT -- I'll Have Another's trainer Doug O'Neill just spoke at a news conference, claiming the horse had developed tendonitis in its left front leg ... and that's why IHA was pulled. According to the trainer, the horse is now officially RETIRED.