"I will expose all that we have kept as secrets so far if they go ahead with the plan to form a new party," Sirisena warned while addressing a rally in the southern town of Matara to mark the first anniversary of the unity government.
"I will tell them that they would never be able to topple this government by forming a new party. I will make sure that they remain on the road, not in government," he said.
Rajapaksa, the 71-year-old former President, handed Sirisena the SLFP leadership after he lost to him in the election held in January, 2015.
A section in the SLFP, who remain loyal to Rajapaksa and do not accept Sirisena's party leadership, now form the joint opposition and are thought to be contemplating a move to form a new party.
They urge Sirisena to form an SLFP government by ending the party's political marriage with archrival United National Party (UNP) of Prime Minister Ranil Wickremesinghe.
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