Phone hacking started at The Sun newspaper before being transferred to News of the World, the brother of former showbiz reporter Sean Hoare told the Leveson Inquiry on Monday.
Stuart Hoare said his brother claimed his ex-editor Andy Coulson was "well aware" of the practice of illegal voicemail interception at the paper - something Mr Coulson strongly denies.
"The reality was that phone hacking was endemic within the News International group (specifically Sean identified that this process was initiated at the Sun and later transferred to the News of the World) and he went on record both verbally and in writing to make this claim," The Guardian reported from the hearing.
Earlier, Hoare said he'd "shared a lot of secrets" with his brother, Sean, before his death, aged 48.
Mr Hoare, who suffered from alcoholic liver disease, was found dead at his home in Watford in July after he started drinking again to cope with the stress of the attention on him as a phone-hacking whistleblower, his inquest heard last month.
His brother Stuart told the Leveson Inquiry on Monday: "We shared a lot of secrets and I felt very, very strongly that someone had to represent my brother." He added that he wanted to "let his word still be heard".
When asked by Carine Patry Hoskins, counsel to the inquiry, what he knew of the "dark arts" of journalism, Mr Hoare replied that he did not witness any incidents, but possessed details of them.
He said: "I was fortunate enough to retain certain information that Sean had left with me. Sean and I regularly discussed this and there are emails in existence which support Sean's description of a practice referred to as the dark arts."
Mr Hoare told the inquiry into press standards that he had a "special relationship" with his younger brother.
He said: "Sean and I had a very close relationship. I think a lot of that was due to the fact we were very different people."
He said that while he was interested in mathematics and sport, Sean "went down the track of drama and the written word".
"We had a very, very special relationship. We spoke probably most days," he added.