Top candidate endorses 'arch-rival' in Lebanese presidential saga twist
Media earlier cited Melhem Riachy, a media official in Geagea's Lebanese Forces Party, as saying Geagea would state his support for Aoun's candidacy at a news conference later on Monday.
The move offers a possible exit to the 20-month vacancy at the country's top post which has left the government in paralysis, boosting Aoun's chances of filling the presidency, but does not guarantee him the post.
Beyond his existing allies, among them the Iran-backed Hizballah, Aoun needs the backing of other groups to secure the necessary parliamentary backing.The Lebanese president, who must be a Maronite Christian, is elected by parliament. Geagea's move was a rare show of unity in a Christian community riven by political divisions for years.
Geagea and Aoun took up arms against each other in the 1975-90 civil war in the so-called "war of elimination".
Geagea's move may also kill off an initiative tabled by his ally the Saudi-backed Sunni leader Saad al-Hariri that would have resulted in another Maronite politician, Suleiman Franjieh, becoming head of state in a power-sharing agreement.
Aoun and Geagea, head of the two largest Christian parties in Lebanon, both opposed the Hariri initiative.
The Hariri initiative is widely thought to have angered Geagea, who had until that moment been the stated presidential candidate of Hariri's March 14 alliance forged in 2005 from groups opposed to Syrian influence over Lebanon.
Aoun is part of the rival March 8 alliance that includes Hizballah and the influential Amal Movement led by Parliament Speaker Nabih Berri.