A Sri Lankan court has stayed disputed Prime Minister Mahinda Rajapaksa and his ministers from holding their positions as it hears an appeal against them.
The decision by the Court of Appeal on Monday is the latest setback to Sirisena and Rajapakse, whose attempt to take power almost two years ahead of the next elections plunged Sri Lanka into crisis.
Rajapaksa said he will take his appeal to the Supreme Court on Tuesday and seek an order halting the decision.
A writ petition was filed by UNP leader Ranil Wickremasinghe and 121 other Parliamentarians seeking an order in the nature of Quo Warranto declaring that Mahinda Rajapaksa is not entitled to hold the office of PM.
"The president has to appoint me prime minister as I command a majority in parliament", he said.
The judge issued the interim order against Mr Rajapaksa and his cabinet, and asked them to appear in court on December 12 to explain on what basis they hold office.
The court order also came as Mr Rajapaksa was set to present an interim budget for the first few months of 2019.
Wickremesinghe - who was unceremoniously dumped by his former ally Sirisena in a late-night power swap - says his sacking was illegal, and parliament supports his stake to lead the country.
President Maithripala Sirisena, who appointed Rajapakse, dismissed parliament last month but the Supreme Court intervened to restore it.
The decision puts a further brake on Mr Rajapaksa's disputed Government after Parliament voted last week to halt payment of ministers' salaries and travel expenses.
Sri Lanka's toppled premier likened the president to Hitler on Tuesday as he demanded to be reinstated, nearly six weeks into a paralysing political crisis in the Indian Ocean nation.
Mr. Wickremesinghe, with the support from the main Tamil party, claims to have the support of more than 113 legislators, required for simple majority.
The UNF has moved three no trust motions against Rajapaksa, who has refused to step down.