Envoi Allen collapsed and died after finishing ninth in the Boodles Cheltenham Gold Cup.
The 12-year-old was having the final start of his glittering career in the Cheltenham Festival highlight, with owners Cheveley Park Stud having confirmed he would retire following his run in the blue riband - his eighth appearance in all at the Festival.
Envoi Allen won 10 Grade Ones in total, having started out with Gordon Elliott before moving to Henry de Bromhead, most recently becoming the first horse to win Down Royal's Champion Chase on three occasions.
Cheveley Park Stud director Richard Thompson said: "We didn't see it. The vets OK-ed them all (after the race) and Darragh (O'Keeffe, jockey) said he was pricking his ears, then he came up and went over as he was coming out of the chute.
"He had just retired and he's just gone in a minute or two.
"He's finished the race, the vets have had a look, he's walked on and he's gone down before the chute."
Envoi Allen had won three times at the Festival, landing the 2019 Champion Bumper, the 2020 Ballymore Novices' Hurdle, now known as the Turners, and the 2023 Ryanair Chase, as well as placing on a further three occasions.
Thompson added: "He was such an unbelievable horse.
"This was not the ending we wanted but unfortunately that is what has happened.
"He's been an incredible servant, wonderful over the years, and this is such a sad way to go.
"He's gone doing what he loves, he just retired at that moment. It's a tough one."
