The opposition slammed the 3.2-trillion-baht budget proposal for the 2020 fiscal year, saying it breaches the law and the constitution.
The House of Representatives began the second reading of the bill on Wednesday, during which opposition MPs in the bill's vetting committee proposed the budget be cut by 5-15%.
They said the proposed reduction covers allocations to agencies which are not entitled to receive allocations from the national budget, particularly agencies which are not legal entities. Such allocations risk breaching budget procedure laws, as well as the constitution, opposition lawmakers said.
Woravat Auapinyakul, a committee member from the opposition, said the budget procedure law stipulates that agencies which are entitled to funding from the state budget must be legal entities.
"However, some 197 billion baht [from the 2020 budget] is being earmarked for funds and agencies which aren't legal entities," he said.
Mr Woravat added the current budget risks breaching Section 140 of the constitution, which governs the payout of state funding.
The section stipulates that state funds can only be disbursed in accordance to the budget law, budget procedure law and budget transfer law.
Some of these agencies are outside the scope of existing budget laws and therefore cannot be scrutinised by parliament, Mr Woravat said.
"I'm concerned that MPs who vote in support of the bill may end up violating the constitution and the law. The National Anti-Corruption Commission has the authority to investigate the matter for 10 years. Minority committee members have issued a number of warnings, only to be ignored," he said.
Anurak Chureemas, a Chartthaipattana MP who is also serving as the committee's deputy chairman, explained that agencies, funds or both which are not legal entities receive their budget from their respective overarching ministries or departments.
Santi Kiranand, a Palang Pracharath Party MP who supports the bill, calls the proposal to slash the budget "impossible".
About 70% of the budget covers routine spending and any reductions will affect money earmarked for investment, he said.
Before the House session began, Pol Gen Sereepisuth Temeeyaves, leader of Seri Ruam Thai, one of the seven opposition parties, threatened to pursue legal action against MPs who voted in support of the budget bill -- regardless of which side they are on.
He said Prime Minister Prayut Chan-o-cha and ministers who proposed the budget bill have no authority to run the country as a result of their incomplete recital of the oath of office -- a stance Pol Gen Sereepisuth has maintained since the controversy broke last year.