Legendary English steeplechaser 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 French-bred 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 was retired from racing after being pulled up in the 2012 Gold Cup having won 23 of his 41 races, 16 at Grade One level.

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.

"It's an honour and a privilege to have been involved with him and had him in the yard," Collett said.

"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."

His owner Clive Smith explained Kauto Star had suffered what appeared to be a minor injury last week but his condition deteriorated over the weekend.

"It's all very, very sad. He was put out to grass, as in fact he always was, even at Paul Nicholls' (stables)," Smith said.

"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.

"He was not responding to treatment on Thursday and Friday and various complications came over the weekend.

"Although he made an improvement and rallied ... on Monday morning it was very bad."

Smith made the decision to euthanise Kauto Star in consultation with the vets.

Kauto Star's trainer Paul Nicholls led the tributes on Twitter.

"RIP my friend you were a true Legend. Once in a lifetime," he tweeted, and attached a picture of the two of them.