Thai Civilised Party leader Mongkolkit Suksintaranont confirmed on Thursday that five small parties with five House seats would leave the coalition government because they had been denied an executive position.
List-MP Mongkolkit said at the parliament the decision to leave applies to his party and the Palang Thai Rak Thai, New Democracy, Thai People Justice and Thai Teachers for People parties.
They had agreed to review their decision to join the coalition, and now planned to be "an opposition within the coalition or an independent opposition", he said.
He said a few months ago Palang Pracharath Party leader Uttama Savanayana and PPRP secretary-general Sontirat Sontijirawong had honoured small parties by inviting them to join the coalition, but the leaders of small parties had not been honoured.
"When the cabinet was formed, we received little response... We do not have a representative on the executive to implement our parties' policies," Mr Mongkolkit said.
"Our role as an independent opposition started two months ago, and dissatisfied the government. Many things make us feel uneasy.
"Whether the government of (Prime Minister) Gen Prayut will last one, four or six months, or four years will depend on whether the leader and the secretary-general of the Palang Pracharath Party can solve this political problem in a managerial way," Mr Mongkolkit said.
"We are neither employees nor subordinates of Gen Prayut, Mr Uttama or Mr Sontirat."
Mr Mongkolkit said their group would vote against the government on issues the government could not explain, and he was waiting for a censure debate.
"If the budget bill cannot pass through the House, the prime minister must resign," he added.
The coalition has 254 House seats. If five small parties withdraw, it will have 249 votes. The opposition commands 246 votes in the House.
The leader of one of the 10 small parties on Thursday was adamant all would remain in the Prayut coalition, despite Mr Mongkolkit's announcement.
Maj-Gen Songklod Thiprat, list-MP and leader of the Thai Nation Power Party, said the 10 small parties remained true to their previous resolution to be part of the coalition.
He attributed Mr Mongkolkit's announcement to his "personal hurt" at not being given an executive position.