Westgarth, 31, beat Dec Spelman at Doncaster Dome on points on Saturday.
Both men hit the canvas throughout the brutal fight and Westgarth needed paramedics in his locker room afterwards.
He was rushed to hospital after collapsing and spent Sunday in medical care but was announced dead this morning.
Promoter Stefy Bull, who organised Saturday’s event, tweeted: “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.”
Eddie Hearn said: “Our thoughts and condolences with the family of Scott Westgarth, may he rest in peace.”
Boxing hero Ricky Hatton said: “Heartbreaking this. Thoughts to his friends and family. And to anyone involved in the show. Rest in peace young man x.”
Former world champion Anthony Crolla said: “Heartbreaking news about Scott Westgarth. My thoughts and prayers and with his family and friends. Rest in piece champ.”
The rest of the event, due to be headlined by Curtis Woodhouse, was cancelled.
Woodhouse, who played for Hull City and Birmingham, wrote on Twitter: “Heartbreaking news about Scott Westgarth . RIP champ.”
Who is Scott Westgarth?
Scott Westgarth was a 31-year-old boxer from Northumberland.
His fight record was seven wins from 10 with two draws and one defeat with a 20% KO rate.
He was a light heavyweight and made his debut in 2013.
Saturday’s victory gave him four wins in a row.
How did he die?
Scott Westgarth beat Dec Spelman at Doncaster Dome on points on Saturday night.
But he was struggling after the fight and needed paramedics to attend to him in his locker room.
And collapsed and was rushed to hospital after looking dizzy and feint in his post-fight conference.
And today it was confirmed he had died in hospital.