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."
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."
Smith hailed his brilliant charge as "the complete racehorse" and explained it was the severity of the injury to his neck which made the decision to put him down inevitable.
He said: "The real injury that has caused the problem is a neck injury, at the base of the neck between C6 and T2 (vertebrae). It affects the spinal chord and in the end, it paralyses through the legs.
"When I saw him yesterday afternoon, he was lying there and I fed him some grass and stroked him and tried to spend the last few minutes with him.
"The decision had to be taken and there was no other course of action to take. Unfortunately he was not able to stand and then he had the pelvic injury."
Reflecting on his astonishing racing career, Smith went on: "He's been a fantastic horse and when you think back about how brave he was, he had the heart of a lion.
"He was so brave, he had everything really. He was the complete racehorse.
"I have so many great memories, after all he won 16 Grade One chases.
"He was an absolutely incredible horse with a lot of speed, winning over two miles in Tingle Creeks, then right up to the Gold Cups, which he needed a lot of stamina for.
"He had a beautiful nature and he will sadly missed by a lot more people than you could ever imagine.
"The response I used to get around the racecourses, people used to come and talk about him all the time. He really has been well loved by everybody and I'm sure he'll continue to be so.
"He just loved the attention, he was a little bit of a show-off. He liked being stroked and given lots of Polos. He was a particularly extrovert type of horse and he was so talented as well, which makes it all the more difficult to take in.
"I just feel so sorry he didn't have chance to have a longer life."