Former England star Michael Owen described the fatal injury suffered by Brown Panther in the Palmerstown House Estate Irish St Leger at the Curragh on Sunday as "the saddest day of my life".
The Tom Dascombe-trained seven-year-old, who was bred and part-owned by Owen, was bidding to repeat his victory of 12 months ago in the Classic on what was his first run since finishing second at Chester in May.
All seemed to be going to plan as he sat in second tracking the pace, but he was suddenly pulled up by jockey Richard Kingscote at around the halfway point, with it later emerging he had suffered a broken hind leg.
Writing on his Twitter page, Owen said: "It's the saddest day of my life. The toughest, most honest, brilliant horse I will ever set eyes on passed away today doing the thing he loved the most.
"A shattered hind leg that was irreparable according to the first class team at the Curragh ended his life.
"I was with him when he was born, shared an experience for seven years that will never be repeated and gave him his last kiss goodbye.
"What an honour to own and breed him. I love you Panther, life will not be the same without you."
Paul Hensey, general manager at the Curragh, said: "Brown Panther suffered a compound fracture of his hind leg. There were three vets with him within seconds and he was put down as unfortunately he just couldn't be saved."
One of Brown Panther's greatest moments came at Royal Ascot in the 2011 King George V Stakes, a win which left Owen visibly moved.
Successful in the Dubai Gold Cup in March, other career highlights included the Goodwood Cup of 2013 and the Ormonde Stakes at Chester and the Henry II Stakes at Sandown, both of which came last year.
He was also a creditable eighth in last year's Melbourne Cup and ran in America and France.
A son of Shirocco, Brown Panther won 11 of his 28 starts, earning over £1m in win and place prize-money.