Westgarth had beaten Spellman via points after 10 rounds in an English title eliminator in Doncaster, South Yorkshire on Saturday night.
Westgarth seemed to be in pain during post-fight interviews and he was rushed to the Royal Hallamshire Hospital where he died.
“To promote a boxing show and a young man doing a job he loves losing his life, I have no words,” Westgarth's promoter, Stefy Bull, confirmed on Monday.
“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.”
Spelman who fought Westgarth on Saturday also took to his social media to express how heartbroken he is over the death.
“'Absolutely heartbroken and lost for words ail continue to pray for Scott's family and the people close to him rest easy my friend,” Spelman wrote.
Westgarth who died at the age of 36, was a former ski instructor and also worked as a chef before he started boxing when he was 24.
His victory over Spelman meant that he was one win away from becoming the English light heavyweight champion.
Other boxing personalities, including Eddie Hearn have all taken to social media to mourn Westgarth.