Hagler had ruled the division from 1979, but was dethroned following a controversial defeat by Sugar Ray Leonard in April 1987.
His wife Kay announced the news on Saturday night, writing on the official Marvelous Marvin Hagler Fan Club Facebook page: “I am sorry to make a very sad announcement. Today unfortunately my beloved husband Marvelous Marvin passed away unexpectedly at his home here in New Hampshire.”
Former WBA and lineal featherweight world champion Barry McGuigan led the tributes, writing on Twitter: “Shocked and deeply saddened to hear of the passing of the incredible Marvellous Marvin Hagler.
“I’m honoured to have spent some amazing times with him. My heartfelt sympathies to his wife Kay and his family. Rest in Peace Champ.”
Eleven-time world champion Oscar De La Hoya said he was “saddened” by Hagler’s death, adding: “One of the greatest to ever step in the ring!”
Promoter Frank Warren said on Twitter: “Boxing lost an all time great today.”
New Jersey southpaw Hagler was only knocked down once during his professional career and defended his middleweight crown 12 times.
In September 1980, Hagler fought Alan Minter at Wembley Stadium, winning by a stoppage in the third round – after the British boxer was badly cut – to claim his first world title.
During 1985, Hagler took on Thomas ‘Hitman’ Hearns at Caesars Palace, Las Vegas, which he won in three brutal rounds of a contest which became known as “The War”.
Hagler took on Leonard in April 1987, a boxer who had retired twice and not fought for almost three years.
Leonard took victory on a split decision, and then retired immediately again afterwards, denying Hagler a rematch in the process.
Hagler retired from boxing in June 1988, with a record of 62 wins, two draws and three defeats.
In 2019, Hagler said he hoped in his lifetime the sport would return to one world champion per division.
He said: “I hope that before I pass that they restore this game and bring it back to only one champion in the world.
“They have these three or four belts, I was only looking for one. Years ago, guys like Carlos Monzon, Bennie Briscoe, Emile Griffith, all of us were fighting for one belt.
“Now you’ve got the WBA, the WBC, the IBF (and the WBO) - they say it’s like the alphabet boys.
“If (a fighter) loses a fight or loses a belt, they can always go to another organisation and try for that belt.
“It should be exciting if they bring back one champion (per division).”
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