The island nation was plunged into a political crisis in October after Sirisena replaced PM Wickremesinghe with former president Rajapaksa, without parliamentary support.
Three lawmakers from Sri Lankan President Maithripala Sirisena's party defected to the government of Prime Minister Ranil Wickremesinghe Tuesday after a dispute between the two leaders.
The island nation was plunged into a political crisis in October after Sirisena replaced Wickremesinghe with former president Mahinda Rajapaksa, without parliamentary support. The move was regarded by Parliament as a "non-violent coup."
Rajapaksa, under investigation for crimes against humanity allegedly perpetrated during the conflict with the Tamil rebels, resigned Saturday after two no-confidence votes and a threat by Parliament to stop payment of cabinet members' salaries.
Wickremesinghe was then sworn in as prime minister for the fifth time.
"We joined the government... as the party and the country have been dragged into a difficult position,” Vijith Wijayamuni Soysa, one of the three lawmakers told parliament.
With the latest defections, Wickremesinghe has the backing of 120 legislators in the 225-member parliament.
Sirisena reportedly said late, at a party gathering, that he would not allow any defector to be sworn in as a minister in Wickremesinghe's cabinet, which is yet to be appointed.
On Thursday Sri Lanka's Supreme Court ruled that Sirisena's decision to dissolve parliament before the end of its term was unconstitutional, a ruling that eventually led to the return of Wickremesinghe as prime minister.
Members of the ruling United National Party, led by Mr. Wickremsinghe, and at least six opposition members of parliament want to begin impeachment proceeding against Sirisena. They would need a two-thirds majority in parliament.
UNP legislator Malik Samarawickrema voiced support for a 20th amendment to the constitution proposed months ago by the People's Freedom Party, which would abolish the Executive Presidency. The Tamil National Alliance and the Sri Lanka Muslim Congress (SLMC) said they will support the amendment.