Westgarth had earned a win on points over ten hard fought rounds against Dec Spellman in an English title eliminator at The Dome in Doncaster on Saturday night.
Scott, 31, suffered a knockdown, but also downed Spellman on his way to a victory considered by many as an upset.
He conducted post-fight interviews but then collapsed in the dressing room afterwards and was taken to Sheffield's Royal Hallamshire Hospital.
His promoter, Stefy Bull, confirmed the devastating news on social media this morning.
He said: "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, its's been the hardest few days I've had to endure.
"No idea what to do moving forward."
Scott, who was trained in Sheffield by Glyn Rhodes and lives in Penistone but is originally from the North East, was knocked down on the ropes just before the bell of round 10.
Speaking to Boxing Social, he said: "I will box anyone. I do it for the fun not because I think I am going to be world class fighter - I just do it for purely for entertainment and I am just glad we could put on a show and keep everyone entertained."
Scott, a former ski instructor, worked as a chef at Sheffield's Royal Victoria Holiday Inn and his victory over Spelman meant he was one win away from becoming the English light heavyweight champion.
He began boxing at the age of 24. Ringside medics accompanied Westgarth to hospital which forced the cancellation of the rest of the fight night.
Former Sheffield United captain and ex-British champion Curtis Woodhouse was due to face John Wayne Hibbert in a Commonwealth title eliminator but the fight was postponed.
He posted on Twitter: "Heartbreaking news about Scott Westgarth.
An image of Westgarth in pain during his post-fight interview was seen on social media on Saturday night.
And following Mr Bull's tweet tributes flooded in on social media on Monday morning.
Promter Eddie Hearn said: "Our thoughts and condolences with the family of Scott Westgarth, may he rest in peace."
Mark Vater said: "RIP @scottwestgarth You were a cheeky chappy who was thought of very highly by your colleagues and friends! You will leave a big void. Thoughts are with your family! So sad."
James Hope-Gill said: "So sorry to hear that Scott Westgarth has passed away. Devastating news."
Luke Junior said: "Absolutely Devastated R.I.P @scottwestgarth A True Gent, A True Warrior. It Was An Honour To Share A Ring & Spar With You And Recieve A Cracked Rib At #KNOX Fly Safe Brother Really Didn't Think This Would Be The Final Bell."