Three-time Long Walk Hurdle winner Reve De Sivola has died after collapsing at Kelso on Saturday.
The popular six-time Grade One winner went down after being pulled up before the final fence in the Edinburgh Gin Handicap Hurdle.
And the 11-year-old, ridden by James Reveley and trained by Nick Williams, was unable to pull through.
Williams said: "I am absolutely devastated. The vet has told us he had a heart attack. It's going to be a long drive home and finding another one like him is going to be so difficult.
"Horses like this are almost irreplaceable. I bought him as a yearling for €6,000 at Deauville and never imagined he would have turned out as good as this, earning more than £600,000 in prize money."
He added: "He's the highest-rated horse I've ever had and there's going to be a huge void in my yard."