The Senate committee on corruption investigation will lobby the Supreme Administrative Court to suspend the government's purchase of 3,183 natural gas for vehicles (NGV) buses.
Paiboon Nititawan, an appointed senator who chairs the committee, said the large scale of the procurement plan would leave it vulnerable to corruption.
He was speaking after the committee questioned Bangkok Mass Transit Authority (BMTA) director Opas Phechmanee about the plan.
The cabinet has approved the BMTA plan to buy 3,183 NGV buses for 13.1 billion baht, Mr Paiboon said.
However, the committee plans to petition the Supreme Administrative Court, through the Ombudsman, to order the government to pilot the project first.
It could then evaluate the buses before deciding whether to invest more money in the fleet.
The committee wants the government to improve the project's transparency by purchasing between 300 and 500 buses for the pilot phase, Mr Paiboon said.
Mr Opas, however, said such a move would mean the median price paid for the buses would be revealed, making any future bids prone to price collusion.