Anthony Honeyball was left "absolutely gutted" after Fountains Windfall suffered a fatal fall during routine exercise on Friday.
The eight-year-old was enjoying a light schooling session in preparation for a novice chase at Warwick on Saturday, for which he would have been a very short-priced favourite to claim a third victory over fences and book his ticket to the Cheltenham Festival.
Honeyball told Press Association Sport: "We decided we'd pop him over some poles, just to keep his eye in. We didn't want to do too much with him running this weekend, obviously. Rachael (Green) rode him and there was another girl who comes in to help us with jumping stood watching. I left them to it and by the time I came back to watch the end he was dead."
The trained continued: "He just didn't take off coming to one of the obstacles. Rachael actually ended up in the obstacle, but she's fine,.
"We're not actually sure what happened. Whether he's just been very unlucky and had a momentary lapse in concentration and fallen badly, or whether he's suffered some sort of heart attack, we don't know. At the end of the day, the end result is the same.
"I can't quite believe it, to be honest. I'm absolutely gutted."
Fountains Windfall made a big impression on his chasing debut at Southwell in November, but fell when in contention for a Grade Two at Newbury on his next start and looked in control of the Kauto Star Novices' Chase at Kempton when crashing out again four fences from the finish.
He bounced back with a facile victory in handicap company last month and was as short as 7-1 for the RSA Insurance Novices' Chase at the Festival in March.
Honeyball said: "His rating of 158, for a novice, said it all really. I was so confident about him heading towards the RSA. I wasn't worried about any other horse and it was just about us doing our job and him doing his.
"I think we'll be more gutted in a few weeks' time when we're coming up to Cheltenham. I'm just in shock at the moment.
"We have some nice horses coming through, but it's going to be very hard to find another one like him."