"I want to be clear - I am leaving because my body can no longer put up with the rigours of the sport and therefore my desire to fight is no longer there," Ward said in a statement on his website.
"If I cannot give my family, my team, and the fans everything that I have, then I should no longer be fighting."
Ward, who won the gold medal at the 2004 Olympics, earned a career professional record of 32-0, including 16 wins by knockout.
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His most recent fight was in June when he earned an eighth-round stoppage over Sergey Kovalev to retain the three light heavyweight world title belts he won from the Russian in a contentious points decision seven months earlier.
(Reporting by Frank Pingue in Toronto; Editing by Toby Davis)
