"We came to an agreement on Monday night that he will be the common opposition candidate," Anura Kumara Dissanayake, a lawmaker from the Janatha Vimukthi Peramuna or People's Liberation Front, told reporters.

"We will ensure his victory and the downfall of President (Mahinda) Rajapaksa," he added.

Rajapaksa, who has two more years of his present term to complete, proclaimed an early presidential election Monday and the elections commissioner will set a date between Jan. 11 and Feb. 1 for the poll.

Fonseka, who has been in hectic talks with several opposition parties, including the United National Party that is most formidable in the opposition, has not yet declared himself a candidate, but he has broadly hinted his hat will be in the ring.

"Many loose ends were being tied up and he didn't want to show his hand until the presidential proclamation calling the 'premature' election was published," an opposition spokesman explained.

It was declared at midnight Monday.

Fonseka, as army commander, has been widely acclaimed both by the government and the country at large for the victory that ended nearly three decades of civil war in Sri Lanka and ended with the rebel hierarchy, including Velupillai Prabhakaran, leader of the Liberation Tigers of Tamil Eelam, dead.

Rajapaksa, as commander-in-chief of the armed forces and defense minister, was Fonseka's political boss, but in the weeks prior to Fonseka's early retirement from the military, relations between the two men were strained.

Soon after the back of the LTTE was broken last May, Fonseka was promoted to general and appointed chief of defense staff, the most senior military position in the country, but he was not given command responsibility.

This was resented by the general who wanted to preside over the army's 60th anniversary celebration in October as commander.

Fonseka has also gone public with the statement the president feared a coup and Rajapaksa's brother Gotabhaya, the defense secretary, feared Fonseka was becoming too powerful.