Former News of the World reporter Sean Hoare was found dead at his home on Monday, less than a year after he became the first named journalist to allege that the paper's one-time editor Andy Coulson knew about hacking at the tabloid.
Police said the death was unexplained but not thought to be suspicious.
Hoare, 47, was reportedly battling drink and drug addiction from his years as a high-rolling showbusiness reporter searching for scoops on the London party circuit.
A post-mortem examination was under way on Tuesday.
He first alleged in interviews with The New York Times newspaper and the BBC last year that Coulson, who edited the News of the World from 2003 to 2007 and went on to become Prime Minister David Cameron's communications chief, knew about voicemail hacking.
"It was endemic. It happened," Hoare said of phone-hacking, in an interview with the BBC in March.
"People were scared. If you've got to get a story, you've got to get it, and you have to get that by whatever means. That is the culture at News International," he said.
News International is the British newspaper publishing arm of media mogul Rupert Murdoch's News Corporation global empire, which owned the News of the World but shut it down earlier this month.
Hoare's claims were passed to Scotland Yard last year but they said he declined to give evidence. Coulson has since been arrested and bailed over allegations of phone-hacking and bribing police.
Just a week ago, Hoare made new allegations in The New York Times about journalists making payments to the police, and about the use of "pinging", the illegal use of mobile phone signals to locate people.
Hoare, whose drink and drug problems led to his dismissal from the News of the World in 2005, was found dead in his home in Watford, north of London, the local Hertfordshire Police force said late Monday.
"The death is currently being treated as unexplained, but not thought to be suspicious. Police investigations into this incident are ongoing," said a police statement.
"The man's next of kin have been informed and the family are being supported by police at this sad time."
The Daily Mirror newspaper quoted an unnamed friend as saying Hoare "thought that someone was going to come and get him, but I didn't know whether to believe half the stuff he was saying."
The Daily Mirror is one of the main tabloid competitors of the News International-owned newspaper The Sun.
Hoare's death also came as Murdoch was due to testify Tuesday to British MPs over the scandal.
Newspaper commentators noted that Hoare's health was damaged from the years of drinking and drug abuse as a showbusiness journalist.
"His health never recovered," wrote The Guardian's Nick Davies, who has led the paper's phone-hacking investigation, which uncovered many of the allegations that led to the closure of the News of the World.
Davies added that Hoare was "a lovely man."
David Yelland, a former editor of The Sun, paid tribute on Twitter, writing: "Sean Hoare was trying to be honest, struggling with addiction. But he was a good man.