Sri Lanka's main Muslim party Sunday quit the government and pledged support to the opposition in a move seen as the biggest setback yet to President Mahinda Rajapakse's re-election bid.

The Sri Lanka Muslim Congress leader Rauf Hakeem also announced his own resignation as justice minister and said he would now work for the victory of Maithripala Sirisena, the opposition candidate in the January 8 election.

Hakeem said they left the government because of festering differences over a 2010 law that lifted the two-term limit on the presidency and gave Rajapakse wide powers over the police, the judiciary and the civil service.

"Good governance is the main issue for us," Hakeem told reporters. "We are guilty of compliance (in voting for the 2010 statute), but now we want to redress the situation."

There was no immediate comment from the government. But a ruling party source told AFP the defection of the Muslim party was the biggest blow to their campaign.

Muslims, the second largest minority in the island after Hindu Tamils, account for about 10 percent of the electorate and could emerge as king-makers in January's presidential election if the majority Sinhalese are split down the middle.

Both Rajapakse and Sirisena are members of the majority Sinhala Buddhist community.

Hakeem becomes the second Muslim minister to quit Rajapakse's government after Industry and Commerce Minister Rishad Bathiudeen resigned.

Sirisena himself defected to the opposition last month after giving up his portfolio as minister of health.

The main Buddhist party of monks quit the government before Sirisena's defection and has since supported his bid to topple President Rajapakse, who came to power in 2005 and is South Asia's longest serving leader.

The Tamil National Alliance, the main party representing Hindu Tamils, has not formally pledged support to either of the two main Sinhalese candidates, but has strongly hinted that it will support Sirisena.

That would make it even more difficult for Rajapakse to win an election which he called two years ahead of schedule.

However, Rajapakse has received the support of a radical outfit known as the Bodu Bala Sena (BBS), or Buddhist Force, which has been accused of instigating religious hate attacks in the past two years.

The BBS has formed an alliance with Myanmar’s 969 movement, a Buddhist group linked to anti-Muslim riots in that country.