Sri Lanka's former military chief will challenge his former boss Mahinda Rajapakse, the incumbent president, in the country's presidential election, an opposition party has said.

The Peoples' Liberation Front, or JVP, said on Tuesday it had come to an agreement with General Sarath Fonseka, the former head of the military, and would back him in election to be held likely in January.

"We came to an agreement last night [Monday] that he will be the common opposition candidate," Anurakumara Dissanayake, a JVP legislator, said.

"He has accepted to be the 'apolitical' common candidate who will work towards the abolition of the executive presidency within six months of coming to power," he said.

The JVP previously backed Rajapakse, but it, as well as the main opposition United National Party, have since said they will support any move to oust the president.

Snap election

Rajapakse earlier called the snap election, which is due between January 11 and February 13. Local media said that the poll may take place on January 23.

The president hopes to benefit from the government's popularity following the end of its military campaign against the Tamil Tigers, who had fought for decades for a separate state in Sri Lanka's north.

Fonseka resigned as chief of defence staff earlier in November, following a rift with Rajapakse over who should take credit for the Tiger's defeat.

The government's offensive against the Tigers culminated in May, ending a conflict that began in 1983 and which killed more than 70,000 people.

Rajapakse has also been accused of corruption and of granting key state positions to family members, positions that would be repealed should Fonseka win the election.

Gotabhaya Rajapakse, his younger brother, is the defence secretary, who also played a key role in crushing the Tamil Tigers.