A Lebanese Christian leader has endorsed his rival for presidency in a bid to unite Lebanon's Christian community and break a 20-month political stalemate.

Samir Geagea has withdrawn from the presidential race in Lebanon, declaring his support for his rival Michel Aoun at a press conference on Monday.

The presidency has been vacant since May 2014 after Lebanese politicians failed to agree on a candidate.

Lebanon has a power-sharing system in which the president is a Christian, the prime minister a Sunni Muslim and the parliament speaker a Shiite Muslim.

Although both Christians, Geagea and Aoun are opponants politically. 

The deal struck between the two leading Christian politicians in Lebanon is made more remarkable by the fact that the two men's militias fought against each other in the brutal battles of Lebanon's civil war 25 years ago, since which they have remained adversaries.

Aoun, leader of the Free Patriotic Movement, is a crucial political ally to Shiite Islamist militant group Hezbollah, while Geagea and his Lebanese Forces Party have stood firmly against the group. They also stand on opposite sides on the issues of Israel and Syria.

"I announce after much studying and thought... that we have adopted the candidacy of General Michel Aoun for presidency in a move that carries hope of getting out of our situation toward a more secure and stable life," Geagea said at the joint press conference with Aoun.

He called on his allies to overcome their clear differences and support Aoun "for the higher Lebanese national interest."

The civil-war in Syria and the subsequent arrival of over one million Syrian refugees into Lebanon have arguably overwhelmed the government, particularly as it stands without a president.

Geagea said he hoped the alliance would "move the country out of the current phase it is in to one that is better."