The 31-year-old won the eliminator bout on points against Dec Spelman.
He appeared to be in pain during post-fight interviews and was then taken to hospital after falling ill backstage.
The rest of the show, headlined by Curtis Woodhouse who was due to fight in a Commonwealth light-welterweight title eliminator, was cancelled.
Westgarth, who was the underdog going into Saturday’s bout, was chasing an English title fight.
His previous record was six wins, two losses and a draw.The news was announced by Westgarth’s promoter for the weekend fight, Stefy Bull, who said the boxer was “a young man doing a job he loves”.
Following news of Westgarth’s death, the British Boxing Board of Control (BBBofC) reiterated that it has improved safety in the sport.
“We are one of the most forward-thinking commissions in the world regarding medical aspects,” general secretary of the BBBofC, Robert Smith, said. “Some people don’t like us because they say we are too strict.
“This is a tough, tough sport and we try to make it as safe as possible but you can’t make it 100% safe,” he added.
“That doesn’t take away from the fact that this is a disaster, but more importantly it’s a disaster for his family and that’s the most important thing.”
Canada’s Tim Hague was the last professional boxer to die following a fight, two days after a bout last June.
Scotland’s Mike Towell died from a brain injury following a bout in September 2016.
Last week, Towell’s opponent, Dale Evans announced his retirement, saying his hunger for the sport has been replaced by “worry and fear”.
Source: BBC
