Jockey Liam Treadwell, who rode 100-1 shot Mon Mome to win the 2009 Grand National at Aintree, has died aged 34.
Treadwell returned to the saddle after retiring from professional riding in February 2018.
Tributes have poured in from the racing world, with the rider described as being "polite, funny, kind and brave".
Police attended Treadwell's Shropshire home after his death, which they are treating as unexplained but said there was no third-party involvement.
It was one of the famous race's biggest surprises when Treadwell triumphed on Mon Mome for trainer Venetia Williams, who said he was like part of the family.
"It's a massive shock. I think we all thought he was in a good place now, having been through some tough times in previous years," she said.
"We shared a day that was certainly the best day of my life, and I suspect of his.
"He was here for a large part of his racing career, and not a day went by without him putting a smile on somebody's face."
Racing history and a toothy smile
Mon Mome was the first 100-1 shot to win the Grand National since Foinavon in the freak 1967 race, and there has been no longer-priced winner in the race's history.
Sussex-born Treadwell landed the ride after Aidan Coleman opted to ride stablemate Stan.
Treadwell was treated to a free dental makeover after BBC interviewer Clare Balding joked about his gap-toothed smile.
Balding later apologised for saying he could "afford to get his teeth done"' but he took no offence and she was among those to pay tribute.
"My heart goes out to his family and all his friends. He was the loveliest guy with a great sense of humour," she said.
Treadwell, who was third in the 2015 National on Monbeg Dude, rode 10 winners during the 2019-20 season, all for Shropshire-based trainer Alastair Ralph.
Big wins in Treadwell's career included the Byrne Group Plate on Carrickboy at the Cheltenham Festival in 2013 and the Grand Sefton Handicap Chase at Aintree with Bennys Mist two years later.
In a BBC interview in 2016, he spoke of feeling depressed and struggling with concussion caused by a fall.
"A sports psychologist encouraged me to watch the good days on TV - good times, successes, so Mon Mome and Carrickboy were two I watched and seeing Clare's comments about my teeth at Aintree definitely helped me to smile again," he said.
"We used to call him 'Tredders'. He just got on with it when he was racing," said former jockey Mick Fitzgerald, who won the National in 1996 on Rough Quest.
"You never knew he was there. He wasn't a shouter. He just wanted to do the best he could, and it's just awfully sad for everybody involved."
A short statement issued on behalf of Treadwell's parents, Mark and Lorraine, and brother Nathan read: "We are heartbroken that this has happened.
"We ask kindly that everyone respects our privacy in the coming days so that we can begin to come to terms with our loss."
The Injured Jockeys' Fund and the Professional Jockeys' Association said in a joint statement: "He was a gifted horseman, valued by racehorse trainers for both this and his communication skills.
"He was polite, funny, kind and brave, having spoken passionately and eloquently about his mental health issues both in the press but also in our own 'Jockey Matters' films.
"It is devastating that this has happened, and our thoughts and prayers are with Liam's family, friends and everyone who knew and supported him."
A statement from West Mercia Police said officers were called to an address in Billingsley, near Bridgnorth, Shropshire, following the death of a man.