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.
RIP my friend you were a true Legend. Once in a lifetime. pic.twitter.com/tGBsz5wfZf— Paul Nicholls (@PFNicholls) June 30, 2015
Will never be bettered pic.twitter.com/t1YcMEQpkf— Paul Nicholls (@PFNicholls) June 30, 2015
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."
Owner Clive Smith explained Kauto Star had suffered what appeared to be a minor injury last week but his condition deteriorated over the weekend.
He said: "It's all very, very sad. He was put out to grass, as in fact he always was, even at Paul Nicholls' (stables). He was put out for weeks at a time there.
"On this occasion he was out in the lovely sunshine, I was away at the time. I saw him on Friday and by that time he'd had what appeared to be a mild injury, but things gradually changed.
"The vets report was brilliantly handled by Hattie Lawrence of the Valley Equine Hospital in Lambourn."
Referring to the report, Smith told At The Races: "He was not responding to treatment on Thursday and Friday and various complications came over the weekend.
"Although he made an improvement and rallied, as he always did in races, on Monday morning it was very bad.
"I came back down from Scotland and the decision was taken with Hattie Lawrence to euthanise the horse."
More to follow
Kauto Star factfile:
Sire: Village Star
Dam: Kauto Relka
Foaled: March 19, 2000 in France
Owner: Clive Smith
Trainer: Paul Nicholls
Jockeys since arriving in Britain: Ruby Walsh, Mick Fitzgerald, Sam Thomas and Tony McCoy
Career earnings: £2,375,883
Race record: 23 wins from 41 starts, including 16 victories at Grade One level.
Major races won:
Cheltenham Gold Cup (2007 and 2009), King George VI Chase (2006, 2007, 2008, 2009 and 2011), Betfair Chase (2006, 2007, 2009 and 2011), Tingle Creek Chase (2005 and 2006), Ascot Chase (2008), JNwine.com Champion Chase (2008 and 2010), AON Chase (2007).