Twice Cheltenham Gold Cup winner Kauto Star has retired following a long career, trainer Paul Nicholls wrote in his column on Wednesday.
The steeplechaser's retirement follows British racehorse Frankel's, who was unbeaten in fourteen races and was the highest-rated racehorse in the world at the time.
While Frankel gave up the game at four years old, Kauto raced until he was 12.
"The end of an era has finally arrived. Clive was here this morning and we have taken the decision to retire Kauto Star," Nicholls wrote on a website after having spoken with owner Clive Smith.
There had been suggestions Kauto Star would make one last appearance in the King George VI Chase at Kempton Park outside London on Boxing Day.
"Deep down, we know he has done enough"
Trainer Paul Nicholls
The popular French-bred racehorse, winner of the steeplechasing showpiece at Cheltenham in 2007 and 2009, recovered from a "pretty awful fall" before March's Gold Cup to compete for a hat-trick but he failed to finish this year's race.
Kauto Star racked up a record five King George wins in nine years with Nicholls, including four straight victories from 2006-2009.
"Ever since he won on his debut for me at Newbury on December 29, 2004, the horse has consistently proved himself a class apart," wrote Nicholls.
"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. Deep down, we know he has done enough."