Synchronised, winner of the Cheltenham Gold Cup, suffered a fatal injury when falling at Becher's Brook in the John Smith's Grand National at Aintree, owner JP McManus said.
He said: “He’s been killed, so I believe. I’m sure they’ll make an announcement shortly.”
The tragedy marred a thrilling and emotional victory in the race for the Paul Nicholls-trained Neptune Collonges.
There was a pulsating climax to the four-and-a-half-mile marathon, with Neptune Collonges (33-1) getting up by a nose in the hands of Daryl Jacob to give champion trainer Nicholls his first National victory, and with it surely sealing another trainers’ title.
Jonjo O’Neill’s Sunnyhillboy was the horse denied under Richie McLernon and while that was a disappointment for connections, there was devastating news to come after his stablemate Synchronised’s fatality was confirmed.
Tony McCoy’s mount got loose before the start, but the partnership was soon intact, only to make it as far as Becher’s Brook, the sixth fence.
Malcolm Jefferson’s According To Pete was reported as another fatality later in the race.
Neptune Collonges was becoming the first grey to win the world’s most famous steeplechase since Nicolaus Silver in 1961.
Last month’s Kim Muir winner Sunnyhillboy looked set to triumph rounding the famous Elbow, but Neptune Collonges got up in the last stride.
For a long way it looked as though Katie Walsh could become the first woman to win the race on the well-supported joint-favourite Seabass, but he had nothing left on the run-in and had to make do with third.
Cappa Bleu finished fourth, with In Compliance fifth and last year’s winner Ballabriggs sixth.
A clearly overcome Jacob said: “You can’t beat this. I was on a tough horse and I said to Paul that one day I would ride you a National winner. That was two years ago and now I’ve done it.”
Nicholls said: “He’s probably the best horse we’ve run in the race. He’s got great form, he’s been placed in Gold Cups, he stays, he’s genuine and Daryl gave him a fantastic ride when you analyse where he went. It’s absolutely brilliant.
“It’s blown Nicky Henderson out of the water now (in the trainers’ championship). Any good race would do, but this is the race we wanted.”