The 27-year-old, who won a bronze medal last year in Beijing, was discovered on Monday by his manager Frank Maloney.
A Metropolitan Police spokesman said the death in Bromley, south London, was not being treated as suspicious.
Mr Maloney was taken to hospital after finding the body and it was later confirmed he had recently suffered a heart attack.
However, there is a possibility Maloney had the heart attack last week, and not after making the grim discovery in the flat.
Former world champion Barry McGuigan said: "It is an absolute tragedy. I would describe Darren as a phenomenal talent."
He added: "I believe he would have become a world champion in the professional ranks one day."
Michael Carruth, who won welterweight gold for Ireland at the 1992 Olympics, said: "He had the power, he had the charisma, everything that goes into making a top-class pro, he had the ingredients there.
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