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.