Thomas Chippendale died of a suspected heart attack after winning the Hardwicke Stakes at Royal Ascot for trainer Lady Cecil and jockey Johnny Murtagh.
The four-year-old colt collapsed after passing the line in a dramatic renewal of the Group Two over a mile and a half.
Murtagh said: "He ran his heart out and was pulled up at the line. He lost his balance and I don't know what happened to him.
"It's heartbreaking for everyone involved - it should have been a great win. It's just devastating the way it's finished".
The 9-4 favourite, Ektihaam, also slipped up on the bend and gave Paul Hanagan a nasty fall, but both the former champion jockey and the horse were reportedly unscathed.
More top news
Film fans are set for a 4D cinema experience that lets audiences experience scents, wind, fog, lightening and bubbles in motion seats.
British expats stockpiling chocolate after US confectioner Hershey's reaches deal to stop the sale of British made Cadbury's in America.
The producers of Guinness, Diageo, are one of the companies battling to succeed Barclays as the Premier League's title sponsors