Sri Lanka's Prime Minister Ranil Wickremesinghe faces a no-confidence motion today that could go down to the wire and lead to political instability in the island nation, even if the government manages to scrape a win.

Wickremesinghe leads a national unity government in alliance with President Maithripala Sirisena's party, which has said it would vote against the prime minister, blaming him for failing to prevent an alleged scam in the bond market.

The opposition, which swept local elections last month, aims to drive a deeper wedge within the ruling coalition with the no-confidence motion and bring forward a national election due in 2020.

Wickremesinghe has faced criticism for failing to deliver on economic growth which slumped to 3.1 percent last year, its worst pace since a recession in 2001.

Wickremesinghe's United National Party-led coalition has 107 members in the 225-member parliament and, until now, the support of 42 members of the SLFP, which gave it a comfortable majority.

But if the SLFP votes against the government, Wickremesinghe could lose his majority.

He would then need the backing of minority parties in the opposition to stay in office.