Thomas Chippendale collapsed and died shortly after winning the Hardwicke Stakes at Royal Ascot on Saturday.
The four-year-old colt, trained by Lady Cecil, held off Dandino in a thrilling finish on the meet's final day, but collapsed after passing the finish line.
The 8-1 winner, ridden by Johnny Murtagh, was pronounced dead at the scene after receiving treatment.
Thomas Chippendale had won three races in 10 starts, including last year's King Edward VII Stakes at Royal Ascot.
"He ran his heart out and was pulled up at the line," Murtagh told the BBC.
"He lost his balance and I don't know what happened to him.
"It's heart-breaking for everyone involved - it should have been a great win.
"It's just devastating the way it's finished."
It was the second win for Lady Cecil, whose husband Henry Cecil died his month, at the Ascot meeting.
"It was devastating, but at least it was quick and he wouldn't have felt anything," she said.
"I didn't know what had happened until I came back. I thought everything was all right but it seemed all quiet and I had no idea why.
"The horse had been working so well."
(Reporting by Josh Reich, editing by Ed Osmond)