Hoare alleged in interviews with The New York Times newspaper and the BBC last year that the tabloid's former editor Andy Coulson, who went on to become press chief to British Prime Minister David Cameron, knew about voicemail hacking.
He was found dead early Monday at his home in Watford, north of London, Hertfordshire Police said in a statement.
"At 10:40 am today police were called to Langley Road, Watford, following the concerns for welfare of a man who lives at an address on the street," the force said.
"Upon police and ambulance arrival at a property, the body of a man was found. The man was pronounced dead at the scene shortly after.
"The death is currently being treated as unexplained, but not thought to be suspicious. Police investigations into this incident are ongoing."
The Guardian newspaper said Hoare had long-term drink and drug problems.
Hoare claimed that Coulson knew about the paper's staff eavesdropping on private messages.
"Everyone was doing it," he told the US paper. "Everybody got a bit carried away with this power that they had. No one came close to catching us."
His claims were passed to Scotland Yard but they said he declined to give evidence.
- AFP /ls
