John Fewkes, who weighed 18 stone at his heaviest, was set to box for the vacant WBF International super welterweight title on June 5.
But a potential health risk was flagged following a routine annual brain scan, prompting the 36-year-old dad-of-two to hang up his gloves.
He said: "They (the British Boxing Board of Control, the sport’s governing body in the UK) said there’s nothing to worry about but there’s just been a couple of changes from last year.
"I don’t really know what they are until I speak to a doctor. It’s nothing for anybody to worry about.”
John insisted he was in good health physically and mentally after dropping seven stone since launching his comeback in 2020 following a six-year break.
He added: “I have got two kids, a gym full of kids and adults that rely on me. There’s a lot of kids in our gym that haven’t got anything else.
"We have a saying in there, ‘No one is bigger than the gym’, including myself. If the gym is the only thing they have got going in their lives and I’m not there, or fit to run it, that’s other people’s lives going down the pan.”
John, who was known as ‘Fireball Fewkes’ in the ring, runs Titans Boxing Gym in Gleadless and trains 20 amateur boxers and two home-grown professionals.
He continued: “Whatever the changes are, however small or large, it’s a change.
"My partner and mum and dad said to me ‘We know how much this means to you the second time round, we know how hard you have worked, but you have got to think long-term’.
"If come 50 I have got dementia because the changes have got worse and worse then I could live in Buckingham Palace and it wouldn’t matter because I wouldn’t be here and have the quality of life to spend time with my family.
"I had to do what’s best for me and my family.”
Last month 28-year-old dad-of-two Luis Palmer from Sheffield was placed into an induced coma after being injured during a fight.
He has since regained consciousness.
John, who has two children with his partner Becky Fairfax – Reggie, three, and nine-month-old Aida – plans to continue working in the sport he first took up aged 11.
By his own admission, returning to the ring helped transform his life.
John said: "I was in a bad rut drinking a bit too much and eating too much, it’s just a vicious circle.
"Since I came back I’ve changed my mindset, become a better father, partner, everything.”
John was the last person to beat Sheffield’s former welterweight world champion Kell Brook in the amateur ranks, but admitted to making mistakes as a professional the first time round.
“I had all the talent in the world as a young lad – more than a lot of kids that won world titles – but I didn’t have the right mindset, work rate and attitude,” he said.
While John never reach the heights he’d hoped for in either chapter of his career, he was able to set an example to the young fighters now under his guidance.
"In a way we did what I wanted to set out to do, it helped me and it helped them,” he added.
"At the end of the day I’m here, I’m healthy and I’m happy and that’s the main thing in life.”
John continued: "I’m so happy with my family, my gym. I can always think it didn’t go to plan, but plans change.”
The one and only comeback fight for Fireball Fewkes took place at a Sheffield Arena car park show last year.
It may not have been as glamorous as boxing inside the venue itself, but some of his demons were at least laid to rest.
"I’ll always remember that night,” he said.
"That will be up there with one of the greatest memories I have had in boxing and I have boxed on Sky and boxed for England.”
‘I have just got to be happy’
John got himself in ‘the shape of his life’ for four subsequent bouts, only for all of them to fall through – including two just 24 hours before the first bell.
"It’s as if someone was telling me or looking after me," he said.
"I have just got to be happy and try to be proud of my career, like everyone says.
"Hopefully I’ll get to a stage where I’m proud of the way I have changed my life over the last two years.”
For many former professional sportspeople the biggest battle lies in retirement and the void it brings.
John’s good friend Kell Brook, whom he briefly trained in 2018, admitted he doesn’t yet know what lies ahead for him after announcing his career was over.
How will he cope?
“I can live without having the closure this time round because I feel I have got the tools to deal with it better,” John said.
"I feel a million times better, I have found so many different coping mechanisms. If things are getting on top of me I go for a run or hit bags whereas before I made the call to KFC or the pub.
"That just doesn’t work, it makes things 10 times worse.”
John added: "I’m going to try and chuck myself into something else, find another challenge.
“Maybe I was never destined to be a world champion, maybe I was destined to make one.”