The former Wimbledon striker made the shock admission a week after it emerged two top footballers – including an England star – plan to come out .
In his first in-depth interview about brother Justin since his tragic suicide in 1998, he said: “I begged him, I threatened him, I did everything I could possibly do to try and stop him coming out.
“I gave him the money because I didn’t want the embarrassment for me or my family. Had he come out now, it would be a different ball game.
“There wouldn’t be an issue, but there was then. Things are different now. Now he’d be hailed a hero.”
Heartache: John wishes he'd reacted differently
Justin shot to fame as a teenager with a stunning goal for Norwich in 1980 against a Liverpool team that included Alan Hansen and Kenny Dalglish.
Brian Clough lavished a £1million transfer fee – a record for a black player – on the England under-21 international to take him to Nottingham Forest. But he did not hit the heights expected.
In a newspaper interview in 1990, he claimed to have had an affair with a married Tory MP he met in a gay bar.
Tragic: Justin took his own life
John, 53, said: “I’ll never forget when Justin first told me. He called me in the evening time and said to me: ‘I’m gay’.
"Then he said to me: ‘I’m planning to go to a newspaper’. I said to him: ‘Oh heavens forbid... oh my God. We don’t need that. You’re mad’.
“He promised when I gave him the money he would not go out and say that. Two days later... bang... headlines in a newspaper. I looked like a sucker.
“For me and my family it was like Hiroshima or Nagasaki on our lives. It knocked us dead, it was a total shock.
“People might not like it, but I was trying to protect my family.
“You’ve got to remember the public’s perception of homosexuality at that time was that it was an abomination . It was taboo. Street boys were beating up gays in nightclubs.
“I give him credit for having the courage to come out and say it. But it caused a lot of confusion and animosity towards him, me, and my family.
“During matches, 30, 40, sometimes 45,000 supporters sang at me: ‘You’re big... you’re black... your a*** is up for grabs... Fashanu... Fashanu’.
“As a result of him saying what he said, my mother died because of the stress. She actually died a year later on the day of his birthday.
"She was already old, very fragile and suffering cancer.
“Then to be told her second eldest son was a homosexual was too much.”
Brothers: John and Justin as young boys
John condemned Justin, then at Leyton Orient, soon after he came out and infamously said he would “not want to play or even get changed” in the same dressing room.
But last week John said: “I’ve never spoken about these things before because I was stamped a homophobe.
"But things have changed and I make it very clear: I was wrong. It was ignorance on my behalf.
"I didn’t understand him. I was trying to protect my family and I was worried about the effect on my career.
“In the process I lost my brother and I am very sad about that.
“He committed suicide because he was so distraught the world would not accept a black man who was homosexual.
"I couldn’t understand it and I couldn’t accept it and so whatever relationship we had before was killed .
“Previous to that, we had been very close, but it just shattered everything. I didn’t speak to him again. There’s a lot of sadness and regrets.”
Change of heart: John has urged fans not to react how he did
The Gladiators presenter said of the two football stars preparing to come out as gay , which the Daily Mirror revealed last week: “If these young men feel it is the right time to come out and announce they are homosexual, please don’t anybody victimise them.
“Please be careful with words, don’t let it lead to the destruction of two men in their prime. I didn’t have that wisdom 20 years ago and it led to the destruction of my late brother Justin.
“We must accept them. I beg everybody not to make the same mistakes I made. Give them as much understanding as possible.”
John, who still believes his brother was not actually gay, is a practising Christian.
He credits his TV presenter daughter Amal, 27, for helping him to change.
She presented a BBC documentary about gay footballers in 2012.
“My daughter Amal is a very strong activist for gay rights,” said John, who now lives in Nigeria, where homosexuality is still illegal.
“She has helped me a lot to understand a lot more about another way of life, which homosexuality is.
"I love her and I am very proud of the way she speaks about her uncle.”