Abdelbaset Ali Mohmet al-Megrahi, the only person convicted over the 1988 Lockerbie bombing over Scotland in which 270 people were killed, died on Sunday, his brother told AFP.
"He died an hour ago," Abdelhakim al-Megrahi said, putting the time of death at shortly after 1.00 pm (1100 GMT).
Doctors had yet to determine the cause of death, he added.
Megrahi, 60, suffered from prostate cancer and was hospitalised for a few days in April before being sent back home to be with his family.
On April 16, Abdelhakim had said his brother's days "were numbered."
A Scottish court in 2001 convicted the Libyan of the 1988 attack on Pan Am flight 103 over the town of Lockerbie, but he was released on compassionate grounds in 2009 after doctors said he had only three months to live.
Megrahi had been greeted as a hero on his return to Moamer Kadhafi's Libya, after having served eight years of a minimum 27-year sentence for his role in the bombing.
The fact that he had survived much longer than the doctors had estimated had provoked indignation in Britain and the United States.
On the second anniversary of the release of the former Libyan intelligence agent on August 20, 2009, the Scottish government insisted its decision to free him had been vindicated.
But British Prime Minister David Cameron criticised the release as a "terrible mistake," and Deputy Prime Minister Nick Clegg said he would like to see him "back in jail behind bars."
Most of those killed in the bombing of the Boeing 747 jet headed from London to New York were Americans. All 259 passengers and crew were killed, along with 11 people on the ground.
Megrahi had always maintained his innocence.© ANP/AFP