The 11-year-old, who carried Warner’s yellow and black colours to victory in the 2003 Champion Hurdle, is believed to have suffered a heart attack when cantering at trainer Philip Hobbs’ gallops at Minehead.
The gelding was being prepared to run in the Stan James Christmas Hurdle at Sandown on St Stephen's Day.
Hobbs’ assistant Johnson White, who was in the saddle, sustained a broken leg.
Warner said: “It’s very sad. I believe it was a heart attack and the only good thing is that he didn’t suffer.
“It happened when he cantered up Philip’s gallops. He collapsed, wobbled and went down. The vet was on the spot but he was dead before he got to him so at least he didn’t suffer.
“He was a wonderful horse and put us on the racing map.
“He won one Champion Hurdle and was second in another, he was runner-up twice in the Tote Gold Trophy and won the County Hurdle.
“He was so popular because he was a grey and he had a lovely name. He was also a trier, he never gave up.
“I had him at home for three months every year and he was an absolute Christian. You could go up to him in the field and put your arm around him.
“Yet he was so competitive and so strong that we couldn’t retire him. He would have got bored.
“He was a horse that took us everywhere.”
Hobbs described Rooster Booster as “an absolute star”.
“He was just having a canter before he had his last serious gallop before the Christmas Hurdle and unfortunately he dropped dead,” he told Racing UK.
“Johnson has broken a bone at the top of his leg.
“Rooster Booster has been with us since he was a five-year-old. He was a very good racehorse and he was a very nice character and so kind.
“He tried in all his races and he enjoyed it. That’s why he was still in training. It’s very sad it’s happened today.
“I’m not sure he was quite as good as he was. He was 11, what do you expect. But at the same time he was still enjoying being in training. That’s why we kept him going.”
“Poor old horse. He’s been an absolute star for us.”
His regular partner Richard Johnson was devastated on hearing the news while on his way to this afternoon’s meeting at Fontwell.
“It’s a big shock. He was a horse that loved life and he was always in a rush to get from A to B,” said Johnson, who rode him to seven of his 10 career victories.
“He gave great pleasure not only to myself but to the owner and to everyone connected with him.
“He gave me one of the highlights of my career when winning the Champion Hurdle. It was a fantastic day and I’ll never forget him.”
The Riverwise gelding graduated from the handicapping ranks to lift the Cheltenham Festival feature two years ago.
Rooster Booster had run with credit in a string of big races before landing a deserved success in the County Hurdle at the Festival in 2002.
He proved a revelation the following season, notching up a sequence of five wins including the Bula Hurdle before landing the Champion Hurdle itself.
Hobbs’ charge won the 2004 Champion Hurdle Trial at Haydock before putting up an amazing performance to be beaten just a short head in the Tote Gold Trophy at Newbury despite shouldering top weight.
Rooster Booster failed to hit his previous heights last season but won on his seasonal bow at Huntingdon before disappointing in the Greatwood Handicap Hurdle at Cheltenham last month.
As well as winning 10 times, Rooster Booster was second in 14 of the 46 races he contested and amassed £687,541 in prize money.