The People's Party leader said it will continue to lead the opposition as the biggest party in parliament and would not back the current coalition ahead of Friday's House vote on a new premier.
Natthaphong Ruengpanyawut said on Thursday that the People's Party would not vote for Pheu Thai Party's Chaikasem Nitisiri, a former attorney-general and justice minister who is a possible candidate for premier.
The People's Party is the latest incarnation of the Move Forward Party (MFP), which was dissolved by the Constitutional Court last week. The court ruled that Move Forward was guilty of jeopardising the constitutional monarchy and national security.
Pheu Thai members will meet on Thursday afternoon to decide which of its two eligible candidates to nominate.
On Wednesday, the charter court found Srettha Thavisin guilty of a gross violation of ethics for appointing ex-convict Pichit Chuenban as a Prime Minister's Office minister. The ruling resulted in Mr Srettha being dismissed as prime minister after less than a year in office and also terminated all positions in the cabinet.
Meanwhile, Palang Pracharath senior member Santi Promphat said on Thursday that his party will support coalition partner Pheu Thai's candidates for prime minister.
Palang Pracharath, which was the political vehicle of the country's last military government, is the third-largest in the alliance with 40 house seats. The party is led by Gen Prawit Wongsuwon, who could potentially be one of the contenders.
