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
One person has been killed and three others injured in a shooting at the Northern Arizona University.
Teejay Stainer's wish was for the rapper to see his prosthetic "Eminem legs", and it was granted after an online appeal went viral.
ITV News Correspondent Lucy Watson is visiting North Korea and offers a glimpse of what ordinary life is like inside the secretive state.