Wickremesinghe says his dismissal was illegal as he commands a greater majority in parliament than Mahinda Rajapakse, the former strongman president controversially installed in his place.
The TNA has already said it would vote against Rajapakse in a no-confidence motion when parliament reconvenes.
Both rivals have been marshalling numbers behind the scenes, worrying civil society groups who protested Sunday in the capital Colombo against the horse-trading.
Wickremesinghe's United National Party (UNP) risks losing legislators to Rajapakse's camp as the days drag by.
Some members of his party have alleged they were offered portfolios and huge amounts of money to switch allegiances.
Seven MPs have already switched sides and been granted portfolios under Rajapakse's new administration.
The state-run Sunday Observer, now controlled by Rajapakse loyalists, defended the change of government but acknowledged the crisis should be resolved.
According to latest counts, Wickremesinghe now has 102 MPs from the 225-seat assembly after another defection while Rajapakse and Sirisena also have 102.
Most of the remaining 21 MPs -- including the TNA -- are set to oppose Rajapakse, observers said.
Meanwhile, thousands of supporters continue to occupy Wickremesinghe's official residence saying they would resist any move by Sirisena to evict their prime minister.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)