The current suspension of parliament by the president is seen as a key to Rajapaksa negotiating for enough defections.
According to latest counts, Wickremesinghe has 103 MPs while Rajapakse and Sirisena together have 100.
Sri Lanka is in a constitutional crisis, torn between two leaders, both of whom claim to be its legitimate prime minister. The alliance had "decided to vote in favour of the no-confidence motion against Rajapaksa", the statement added.
Kehaliya Rambukwella, a spokesman for the new government, said it would renegotiate the 99-year lease for the port, which Rajapaksa's supporters deride as an outright sale. "The informant has made a statement regarding a Cabinet minister involved in the conspiracy to assassinate me".
At least 4 more of the TNA's 16 MPs are believed to be to joining Rajapaksa.
"We strongly oppose and condemn Tamil National Alliance Member of Parliament being a part of this conspiracy", TNA said.
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The president suspended parliament for 20 days until November 16 after sacking Wickremesinghe, in a move to put off a parliamentary vote that would have gone against his choice for prime minister. Sirisena has been sending conflicting signals about his willingness to end the parliamentary suspension.
Sirisena was also critical of investigations into military personnel accused of human rights violations during Sri Lanka's long civil war, which ended in 2009.
Some members of his party have alleged they were offered portfolios and huge amounts of money to switch allegiances. "I have fears it won't be the one we agreed on", Margue said. President addressing the nation, recently, said the plot to assassinate him had prompted him to appoint former President Mahinda Rajapaksa as the PM and confirmed the involvement of a Cabinet Minister too in this plot.
Rajapaksa's government also came under fire for taking billions of dollars in loans from China to build a scantly used port.
Kabir Hasheem, the UNP chairman and, until last week, the government's minister of highways and road development, said other lawmakers described similar offers, but most MPs were not selling out.
The EU warning on trade is the strongest yet from Western powers which, along with India, the island's nearest neighbour, have urged Sirisena to abide by the constitution. Necessary action will be taken against him immediately.