The pro-Syria Lebanese general Michel Aoun met Israeli Mossad agents on several occasions while in exile in Paris between 1991 and 2005, the French daily Le Figaro reported on Friday.
The report came as Aoun, the leader of the Free Patriotic Movement, was visiting Damascus.
The Christian ex-military chief, who was forced into exile after a failed coup against the Syrians, has since aligned himself with Damascus and Hezbollah.
The report also said that the French intelligence authorities warned Aoun against these meetings, but he ignored their advice.
Aoun turned the page on a turbulent past with Syria on Wednesday in a visit to Damascus condemned by rivals in Lebanon who still see the neighboring state as a threat to its sovereignty.
Syrian President Bashar Assad greeted Aoun warmly at his hilltop palace in a meeting underlining the dramatic shift in the position of the former general who was defeated in battle by Syrian forces at the end of Lebanon's 1975-90 civil war.
"We spoke with our hearts and minds ... so there remains no trace of a past in which there are many painful things," said Aoun, who heads the largest Christian bloc in parliament.
"Our meeting today is a promise of a bright future," he added at a news conference afterwards.
Buthaina Shaaban, an adviser to Assad, said Aoun's visit opened "a new era in relations between Syria and Lebanon".
Aoun holds no official post, but he is an influential figure in Lebanon's sectarian politics and an ally of the powerful Shi'ite Hezbollah, a pro-Syrian faction with a guerrilla army.
Relations between Damascus and Beirut have improved this year after an agreement to calm a bitter power struggle between a U.S.-backed coalition opposed to Syrian influence in Lebanon and an alliance led by Hezbollah and its allies, including Aoun.
Aoun, 73, was prime minister of an interim government when he was forced into exile under Syrian fire in 1990.
He returned to Beirut in 2005 after Syrian troops withdrew from Lebanon following an international outcry over the assassination of former Prime Minister Rafiq al-Hariri.
Syria's opponents in Lebanon accuse Damascus of orchestrating the Hariri killing and other assassinations since his death. Syria denies the charges.
Aoun was a harsh critic of Syria's role in Lebanon from exile in France but says he changed his views once Syrian troops had ended their 29-year presence in his country.