Sri Lanka's main opposition party says it has decided to back General Sarath Fonseka, a former army commander, in the upcoming presidential elections.

Former Prime Minister Ranil Wickremesinghe, who leads the opposition United National Party (UNP), made the announcement at a news conference in Colombo on Thursday.

"We will be considering this election as a referendum to oust President [Mahinda] Rajapaksa and his family from power and thereafter call for parliamentary elections," DPA quoted Wickremesinghe as saying.

Ex-army commander General Sarath Fonseka is widely credited for the defeat of the Tamil Tigers earlier this year.

He retired from his post two weeks after the defeat of the Liberation Tigers of Tamil Eelam that ended a 26-year old civil war in the country.

Fonseka, 58, poses the most serious challenge in Rajapaksa's bid for re-election. The government has not yet announced a date for balloting, but it is likely elections will be held on January 23, 2010.

JR/JG/DT