Kauto Star's legendary career has come to an end with the dual Cheltenham Gold Cup winner, described as a "once-in-a-lifetime horse" by his trainer Paul Nicholls, officially retired.

Regarded as one of the greatest horses in jump racing history, Kauto Star won the Gold Cup in 2007 and 2009 and made history by claiming a record fifth King George VI Chase last year.

The 12-year-old was pulled up during his attempt at a third victory at Cheltenham in March and owner Clive Smith and Nicholls decided to put him into retirement after meeting on Wednesday.

"We've decided to retire Kauto Star as of today," Smith told BBC Sport.

"I'd always thought he had done enough. He's had a magnificent career and I'm mighty proud of him.

"It's been the journey of a lifetime. He's a wonderful, wonderful horse and a good friend of mine."

Nicholls said in his Betfair column that the horse was bright and well but it was the right time to end his career.

"The end of an era has finally arrived," Nicholls said.

"Don't get me wrong, Kauto looks and feels as vibrant as ever, as those who saw him at our owners' open day last month would testify to. He was mad fresh that day and continues to be as alert as ever and very, very well in himself.

"But I suspect Kauto will be like that when he is 20-years-old."

Kauto Star originally looked set for retirement in 2011 after being pulled up at Punchestown.

But, after several months away from racing, he returned to win at Haydock in November before his record-breaking success in the King George.

"Kauto is signing off in full health and after a season in which he proved so many people wrong by winning a fifth King George and a fourth Betfair Chase, to add to his two Cheltenham Gold Cups and Tingle Creeks," Nicholls said.

"Who wouldn't want to train a horse of this quality any more? He is a once-in-a-lifetime horse, after all," he added.

"Ever since he won on his debut for me at Newbury on December 29, 2004, the horse has consistently proved himself a class apart.

"But sometimes, you have to listen to your head and your heart, and both told me that retirement was the only option."

Kauto Star leaves racing as the earner of almost STG2.4 million ($A3.75 million) during his stellar career.