Anti-corruption advocates have demanded the government and Royal Thai Navy disclose information on its "secret" submarine purchase to ensure the transparency of the deal.
The call came as the navy reportedly is set to sign the purchase contract with China early next month.
Anti-Corruption Organisation of Thailand (ACT) secretary-general Mana Nimitmongkol said the government's decision to buy a 13.5-billion-baht Yuan-class S26T submarine from China has attracted widespread public criticism.
The administration should reveal the details of the pricing and purchasing process, including whether the submarine would be bought directly or through a middleman, Mr Mana said, adding it also needs to spell out the total budget required.
Although the government has cited security reasons for secretly approving the deal, the issue involves taxpayers' money and disclosing all information would boost the image of the regime, he said.
"I want the government to openly heed concerns and explain the matter rationally without thinking it is being watched for wrongdoing," Mr Mana said.
He was speaking at a forum on the monitoring of the government's anti-graft policy organised at the Thailand Development Research Institute (TDRI).
Duanden Nikomborirak, a TDRI research director, said the public has the right to question and express concerns about the transparency of the submarine procurement and whether it is worthwhile.
If the government reveals the proceedings, including how it selected the supplier, the public would accept it, she said.
She rejected the government's claims over security concerns, saying some information can still be revealed to the public.
A navy source said navy chief Adm Na Areenich would fly to China to sign the contract early next month.
Adm Na meanwhile said the navy is in contact with China in preparation to ink the submarine purchase contract "soon", refusing to confirm specific details.
The navy's procurement of the vessel from China was approved in secret at a cabinet meeting on April 18, with the navy to pay for the submarine from its own coffers. However, the approval was only made public on Monday after the media delved into the matter.
In the entire procurement plan, the navy will pay for three Yuan-class subs costing a total of 36 billion baht in instalments over 11 years.
The government came under criticism for apparently covering up the cabinet approval despite the fact the issue is public knowledge.
The administration earlier insisted there is nothing dubious about the project and, since the documents shown in the cabinet meeting were confidential, it claimed it is normal for the government to avoid making the matter public.
Speaking Wednesday after a meeting of chiefs of the armed forces, Supreme Commander Surapong Suwana-adth said the procurement is under way and when it is completed, the navy will hold a news conference about it.
"Regarding information on a submarine purchase like this, as per international standards, this information must be confidential. Not much of it can be revealed, except the broad picture of the programme," Gen Surapong said.
He insisted the submarine programme complies with the country's defence strategy. "The decisions have been made now, but it will take almost 10 years before all subs are obtained," he said.
Gen Surapong was tight-lipped when asked to respond to Deputy Prime Minister Prawit Wongsuwon's assertion that China would supply submarine-launched missiles for free, saying this kind of issue is not usually discussed.
This is because other countries could buy weapons to outgun these missiles, Gen Surapong said.
The supreme commander insisted the submarine purchase programme would be a boon to the national interest, citing the uncertain global geopolitical situation which he said could change drastically over a short period of time.
Political activist Srisuwan Janya, secretary-general of the Association for the Protection of the Constitution, petitioned the Office of the Auditor-General, asking it to check the transparency of the submarine procurement programme.
He said the public had questions over the feasibility of the scheme as the Gulf of Thailand is considered too shallow for submarines.