The navy is pushing to pay in yearly instalments for the two submarines it is buying, saying it has a duty to propose the spending despite heavy criticism.
The national budget expenditure bill, before the House of Representatives, includes the navy's contentious plan to purchase two subs from China worth 22.5 billion baht.
The purchase is part of the wider 200-billion-baht defence budget being considered by a House scrutiny panel following the bill's second reading.
The first payment was supposed to have been approved last year but was postponed so the money could be diverted to fight the Covid-19 pandemic.
The navy is back at the House to defend the spending again. Navy spokesman Chettha Jaipiam insisted the force was duty-bound to do so.
"It's a matter of procedure," Adm Chettha said, noting that whether the spending will be passed or not is a separate issue. "There is no ulterior motive," he said.
The navy was aware of opposition to the defence budget and its plan to acquire the subs.
"We face resistance whenever the issue is brought before the committee. It's not the first time we've been given a dressing down," Adm Chettha said.
If the lawmakers insisted the procurement was premature, there was nothing the force could do. But the spokesman added that when the navy has formulated a plan to develop its defence capability, the force must see to its implementation.
He said any disruption to the procurement would affect the submarine fleet structure as 2-3 subs are needed to be commissioned in case one comes in for maintenance and repair.
In the next fiscal year's budget, the navy has negotiated with China to lower the first instalment by one-third. Details will be presented to the House scrutiny committee and released to the public later. Pheu Thai Party MP Yuttapong Charasathien said the leaders of the armed forces did not testify on the defence budget allocated.