A post-mortem will be carried out to establish why Many Clouds, the 2015 Grand National winner, collapsed and died moments after achieving a heroic victory here on Saturday over Thistlecrack, who was previously unbeaten over fences.

A heart attack was suggested as the cause by the trainer of Many Clouds, Oliver Sherwood, but an animal rights organisation said the horse should not have been allowed to continue racing, having shown symptoms of post-race distress in the past.

“I think it was a massive heart attack,” an emotional Sherwood said. “He wanted to win that race, by God he wanted to win it. He was beat at the last, he fought the last 50 yards to get up and win. He was the horse of a lifetime.”

Sherwood sent Many Clouds for an operation last summer to help his breathing after he was well beaten in last year’s National. The trainer seems to feel that breathing difficulties may have contributed to incidents when Many Clouds appeared unsteady after his races, including after his National success.

A spokesman for racing’s ruling body, the British Horseracing Authority, described this as “post-race ataxia” and suggested it was not normally regarded as posing a risk of death. “It can be characterised by a wobbling walking action after exercise,” said Robin Mounsey. “This is not uncommon for racehorses after exercise and can usually be addressed by cooling the horse with water.

“We cannot yet be certain of the cause of the incident, but steps will be taken to understand exactly what has happened. Incidents such as this are exceptionally rare.”

Dene Stansall, of Animal Aid, said he had emailed the BHA last year to say Many Clouds should be barred from the Grand National. Stansall claims to have written at the time: “The BHA’s duty and responsibility is to prevent him from suffering; or to be held responsible should he collapse and die in any of his future races.”

The BHA, which takes the view that Animal Aid simply wants to put an end to horse racing, declined to confirm or deny receipt of any such email.

The spiritual home of jump racing was shocked into silence by the sudden death of Many Clouds, just moments after one of the sport’s most popular animals had delivered the performance of his career to edge out Thistlecrack in the Cotswold Chase. The joy of a nine-race day full of clues for March’s Festival seemed to evaporate after the news was announced, drawing a sound very much like anguish from the 22,500 crowd, a record for the fixture.

Sent off as the hot favourite at odds of 4-9 after dominating his rivals in the King George VI Chase last month, Thistlecrack cruised through most of this race in a style that would have reassured his supporters. But he could not shake off Many Clouds and was gradually worn down on the long uphill run-in, going down by a head at the line.

It was exactly the kind of hard-fought, slow-motion battle that has helped to make this place so popular over the years and was therefore something to savour for followers of jump racing, even those who lost money on the favourite. But before the implications of the result could even be considered, Many Clouds had collapsed in the pulling-up area about 100 yards beyond the winning post.

Screens were erected around the 10-year-old and vets were quickly on the scene, but the horse could not be saved. Sherwood suspected a heart attack as the cause, suggesting that what happened here was not related to the incidents when the horse was unsteady on his feet in the immediate aftermath of some of his previous races. Sherwood seems to feel now that those were caused, at least in part, by breathing difficulties, eased by last summer’s wind operation that may have allowed Many Clouds to deliver this mighty effort.

The trainer said he had had to have two large drinks before offering his reaction. “And I haven’t had a drink for about a month. It’s virtually IV vodka, just to sort of relax me.

“I always said he’d die for you and he’s died for me and the team today, doing what he loved most. He wanted to win that race, by God he wanted to win it. He was beat at the last, he fought the last 50 yards to get up and win.

“They’re dreams, horses like that, complete dreams. I’m sad for Trevor [Hemmings, the owner] and all my team. I’ve been in the game 32 years, training, now and horses like that don’t come along very often. So you’ve got to enjoy it.

“It happens to us all, we’ll all go one day. You’ve got to be philosophical about it and just enjoy the moments: the National, the Hennessy.

“I want one thing now, I want Thistlecrack to go and win the Gold Cup, to just show it wasn’t a fluke.”

A succession of rival trainers approached Sherwood to offer some comfort, most of them visibly shaken and at least two of them wiping away tears, including Alan King, whose Smad Place finished third. Jockey Leighton Aspell, who rode Many Clouds in all 26 of his races, left the course immediately.

Joe Tizzard, who assists his father, Colin, in training Thistlecrack, spoke for many when he said of Many Clouds: “It’s a sad, sad thing. He ran a hell of a race and then that happens. I can’t imagine how they’re feeling, it’s horrible.”

Colin Tizzard declined to offer excuses for Thistlecrack’s defeat, saying: “We were beaten by a better horse on the day. Two lovely horses ran themselves to the line. Poor Many Clouds. Our initial thoughts when we got beat were, it’s a bit disappointing, but this is as sad as can be. He’s a fantastic, lovely horse.

“Everyone here will feel exactly the same. It’s what happens in our sport occasionally and you’ve got to face up to it.”

Tizzard struck a similar tone before he knew the winner had collapsed, saying that Thistlecrack had been outstayed. “Well done Many Clouds,” he said. “We haven’t been used to this with Thistlecrack and you almost come to expect it [winning]. But they’re racehorses. He’s still run a beautiful race and that might be some of the best form we see this season. Many Clouds is a good horse and he’s beaten us.”

Thistlecrack remains at the top of betting for the Cheltenham Gold Cup in March, but is now available at 5-2 rather than the even-money that was the best price on Saturday morning. Two of his stablemates are among those whose odds have now shortened, Native River being the second-favourite on 11-2 while Cue Card is 9-1 from 12s.

Sunday’s tips by Chris Cook

Fontwell

1.40 Solighoster (nap) 2.10 High Aspirations 2.40 Not Another Muddle 3.10 Golden Birthday (nb) 3.40 Venetian Lad 4.10 Sutter’s Mill

Sedgefield

1.20 Mahlerdramatic 1.50 Volvalien 2.20 Kelka 2.50 Ascendant 3.20 Special Catch 3.50 Tiger Mountain 4.20 Yur Next