Sri Lankan President Maithripala Sirisena on Thursday suspended parliament for nearly a month, an unexpected move that comes as a power struggle deepens between him and his unity government's prime minister.

A decree released by the government quoted the president as stating that, under Article 70 of the island's constitution, he had discontinued parliament “with effect from midnight” of April 12.

In recent weeks, Sirisena has reduced the responsibilities of Prime Minister Ranil Wickremesinghe, removing from his control the central bank, the policy-making National Operations Room and several other institutions.

Earlier in the day, six ministers quit Sri Lanka's troubled coalition government. The prime minister's United National Party had increased pressure on the followers of President Maithripala Sirisena — who voted against Wickremesinghe in a recent no-confidence motion — to resign.

With the help of minority Tamil and Muslim parties, the premier defeated the motion, dealing a blow to Sirisena, who had campaigned for Wickremesinghe to stand down.

Those who voted against Wickremesinghe “have no moral right to remain in government”, Petroleum Minister Arjuna Ranatunga said.

Relations between the rival groups in the unity government soured after both suffered losses in February's local council elections.

Former strongman Mahinda Rajapaksa's party trounced the UNP and SLFP to win control of two-thirds of the 340 local councils at stake.

In the run-up to the election, Sirisena branded the prime minister and his party as more corrupt than the Rajapaksa regime, which Sirisena and Wickremesinghe jointly toppled in 2015.