Trained by Colin Tizzard and owned by Jean Bishop, Cue Card was a dual Cheltenham Festival victor in winning the 2010 Champion Bumper and the 2013 Ryanair Chase and retired in 2018 with a career tally of 16 wins in 41 career starts.
Cue Card was due to lead the parade of this year's King George at Kempton on Monday, but Tizzard's son Joe, who now holds the training licence, announced the death of the hugely popular 16-year-old.
Speaking on behalf of the Tizzard family, he said: "Sadly Cue Card had a heart attack when out for his regular exercise this morning, and died immediately.
"He was an incredible racehorse for Bob and Jean Bishop, and for us all, and there's no doubt that without him coming along when he did, things may have panned out very differently here at Venn Farm, as he was the first top-class horse we had, and he showed people that we could handle these Grade One horses, which led to the likes of Thistlecrack and Native River."
Cue Card made a successful racecourse bow in January 2010 when winning a Fontwell bumper by six lengths before springing a 40/1 surprise at the Festival in March.
That proved the springboard for an exceptional racing career, with Joe Tizzard his regular partner until his retirement from the saddle in 2014, with the pair racking up a string of big-race successes, including the Ryanair and the 2013 Betfair Chase at Haydock.
Paddy Brennan then struck up a great relationship with Cue Card, winning two more Betfair Chases in 2015 and 2016 as well as grabbing a dramatic head success over Vautour in the 2015 King George at Kempton.
That victory combined with his Betfair success had Cue Card on course for a £1million jumps Triple Crown bonus in the spring of 2016, but he came unstuck at the third-last fence in the Cheltenham Gold Cup, ending his chance of the mammoth prize.
Tizzard paid tribute to "superstar" Cue Card, who spent his entire career with the team, and nominated the 2013 Betfair Chase success as his own personal highlight.
He continued: "We bought him as a three-year-old store, and we got an early indication of his talent when we worked him alongside some of the summer jumpers just a few months after we had broken him in, and he beat them comfortably!
"His win in the Cheltenham Festival bumper was huge, as it was the first for the yard, and I'd gone 11 years without riding a Festival winner.
"It was a big ask running him that day, but we knew he had the ability, and he proved it in great style. He just needed time to mature after that, so while he was a very good hurdler, chasing was always going to be his thing.
"He was so sound, his longevity was testament to that, and we only ever had a few tiny setbacks, but nothing ever that serious, and that meant he was running in these big races year after year. I was lucky enough to ride the likes of See More Business and Flagship Uberalles, but Cue Card was the best I ever rode, he was a superstar.
"If I had to pick out one victory of his that really stands out, I'd have to say his first Betfair Chase win in 2013. Bob Bishop called dad (Colin) at 5am that morning, as he was having doubts about running him in such a good race.
"So then dad called me to check that we were doing the right thing, which of course we were, as he beat a really deep field. But there was pressure that day, which is why dad was so emotional after the race."
Cue Card made an appearance at the Royal Windsor Horse Show in 2019, taking part in RoR (Retraining Of Racehorses) in-hand show classes, but he remained an important part of the Tizzard yard.
Tizzard added: "For the last three years he has been (my wife) Rachel's hack. This is a sad day for everyone here, he was such a huge part our lives, and he'll never be forgotten by any of us."