Kauto Star has been put down after suffering pelvis and neck injuries in a fall in a paddock.
During a glittering career with champion trainer Paul Nicholls, the 15-year-old won the King George VI Chase at Kempton five times and became the first horse to regain the Cheltenham Gold Cup.
He retired from racing after pulling up in the 2012 renewal of the blue riband having won 23 of his 41 races and winning well over £2million in win and place prize-money.
Kauto Star went on to pursue a career in dressage with Laura Collett and last December took part in a demonstration on the opening night of the London International Horse Show at Olympia.
Collett said: "It's an honour and a privilege to have been involved with him and had him in the yard.
"It's just horrendous. He was out in the field, the same field he was out in every day, we don't really know what happened, but he injured his neck and pelvis and sadly had to be put to sleep."
