Westgarth looked uncomfortable as he was interviewed ringside after his points victory over Dec Spelman in Doncaster and was taken to hospital having deteriorated backstage.
The 31-year-old, who had been chasing an English title fight, died in Royal Hallamshire Hospital.
“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,” Stefy Bull, Westgarth’s promoter, wrote on social media on Monday.
“Absolutely heartbroken and lost for words, all continue to pray for Scott’s family and the people close to him rest easy my friend,” he said on Twitter.