The London Ex-Boxers' Association says Duff, who was born in Krakow, Poland, died in his sleep early Saturday. The association says on its website that "this is truly is the end of a golden era in British Boxing."
Duff, who moved to England in the late 1930s, won 61 of 69 professional fights before retiring from the ring at the age of 19. He went on to manage 16 world champions, including Frank Bruno, Joe Calzaghe, Jim Watt and Alan Minter.
In 1999, Duff was inducted into the International Boxing Hall of Fame, which describes him as "one of the most knowledgeable and accomplished men in the history of the sport."