Trainer Paul Nicholls has spoken of his deep sadness after hearing the news Kauto Star had to be put down due to injuries suffered in a paddock accident.

The history-making steeplechaser suffered pelvic and neck injuries in a fall last week and the 15-year-old was euthanised on Monday.

Kauto Star enjoyed a glittering career with the champion trainer after being bought from France as a four-year-old.

His 16 Grade One victories include five King George VI Chases at Kempton and two Cheltenham Gold Cups.

He was retired from racing in 2012.

"I was mortified when I heard. It's an extremely sad day. He was obviously a big part of the lives of everyone here in Ditcheat," Nicholls told At The Races.

"He was a champion of all champions, really, and I was very lucky to have him in a golden era.

"He was like my best mate. I saw him every day and he was a great horse in every way temperament-wise.

"When he left, it was obviously a big hole we had to fill in everybody's lives. He'd been so good for racing and so good for everybody.

"I've been very lucky to have trained some incredible horses, but I've always said he's once in a lifetime."

Kauto Star's regular jockey Ruby Walsh said he would never see the likes of him again.

"He was without a doubt the best chaser I've ever ridden - a horse in a lifetime. I'll never ride one as good as him again," Walsh sadi.

"Kauto, very simply, was one of the greatest jump horses in history."