Musa Sadr, the founder of the Amal Movement in Lebanon, disappeared in 1978. His movement is led now by Lebanese Parliament Speaker Nabih Berri.
A source from the Libyan Transitional National Council was quoted by The Daily Star newspaper in Lebanon as saying Sadr died of natural causes in 1998.
"Imam Musa Sadr died in his prison cell where he was being held since his disappearance at the hands of (security) members of the Gadhafi regime in 1978," the source said.
The same source said Sadr's body was likely removed from a morgue by figures loyal to Moammar Gadhafi to "cover up the crime." His body was kept in a Libyan morgue until shortly after NATO operations began in Libya in March.
The Lebanese government decided to have its ambassador to Cairo, Khaled Ziyadeh, represent Beirut at the summit for the Arab League held in Tripoli last year.
The Beirut government threatened to boycott the summit in Libya because it blamed Gadhafi for the 1978 disappearance of Sadr. Gadhafi's government said Sadr disappeared after he left the country for Italy.