The 31-year-old had beaten Spellman on points in an English light-heavyweight title eliminator on Saturday before falling ill backstage, having suffered a knockdown late on.
Westgarth was taken to the Royal Hallamshire hospital – he had fought at The Dome in Doncaster – and died in the early hours on Sunday, the British Boxing Board of Control have confirmed.
HOW DID SCOTT WESTGARTH DIE? TRAGIC DEATH EXPLAINED
His points victory over Spellman, which represented the seventh win of a 10-fight career, had been scheduled on the undercard of Curtis Woodhouse-John Wayne Hibbert, which was postponed and then cancelled after Westgarth fell ill.
But, staggeringly, Westgarth was still well enough to be interviewed after the fight.
And in the last recorded footage of the boxer he spoke of how he was glad the fight was entertaining.
Scott Westgarth posted this picture online in the build-up to Saturday’s fight (FACEBOOK)
He told Boxing Social: “I think I controlled the fight. It’s hard sometimes, you’re going to get caught. It’s a good job I’ve got a good chin otherwise he would have seen me away.
“At the end of the day I wanted it just as much as he wants it and I think ultimately I box away all the time and I’m used to the pressure.
“I do this for fun not because I think I’m a world class fighter, I’m glad we could put on a show and keep everyone entertained.
“It’s a case of being able to take one to give one sometimes.
“I don’t really care who’s next. I just do boxing because I enjoy it, that’s the only reason I do it. I enjoy going to the gym with the lads, and I train regardless – win, lose or draw.
“It’s no skin off my shoulders, I just box who I’m told to box. Too many people get carried in boxing, it builds up their egos and makes them think they’re better than they are.
“They get put in tough fights and they get taken to pieces. I came to boxing late and I do it because I enjoy it. Win, lose or draw tonight I would have continued.”
Westgarth was a former ski instructor who worked as a chef at Sheffield’s Royal Victoria Holiday Inn, having lived in Penistone and originated from the north-east.
The show’s promoter Stefy Bull said on social media: “God bless Scott Westgarth. To promote a boxing show and a young man doing a job he loves losing his life, I have no words.
“RIP lad, thoughts go out to your family and your team, it’s been the hardest few days I’ve had to endure. No idea what to do moving forward.”
The board’s general secretary Robert Smith told Press Association Sport: “I’ve spoken to the doctors and officials in charge and am satisfied with the procedures. It’s a terrible thing that’s happened. My condolences to all of his family.”
Tributes have poured in for tragic boxer Scott Westgarth (INSTAGRAM)
Further social media tributes have since come in from the boxing community, including from 26-year-old Spellman, who had suffered his first defeat, and posted: “Absolutely heartbroken and lost for words. I’ll continue to pray for Scott’s family and the people close to him. Rest easy my friend.”
Woodhouse added: “Heartbreaking news about Scott Westgarth. RIP champ”, while Ricky Hatton wrote: “Heartbreaking this. Thoughts to his friends and family. And to anyone involved in the show. Rest in peace young man.”
Carl Frampton posted: “Heart breaking to hear the news that Scott Westgarth has passed away. It’s a stark reminder of how dangerous this sport is. RIP.”
Kell Brook tweeted: “Condolences to the family of Scott Westgarth, My thoughts and prayers are with you all.”