The Democrat Party has demanded the government present documents to prove it has sealed a rice export deal with Beijing, after the Chinese ambassador to Thailand said the agreement had only been reached in principle.
The government has claimed it sold five million tonnes of pledged rice to China under a government-to-government (G-to-G) deal.
Democrat list MP Ong-art Klampaibul said the government must back its claims by producing copies of the G-to-G agreement contracts, customs export documents and receipts.
The opposition will give the government until tomorrow to produce the documents, he said, without stating what would happen if they were not presented.
Mr Ong-art said that even though the government had survived last week's censure vote, the opposition will continue to pressure the administration on alleged irregularities and accusations of graft in the rice pledging scheme and other projects.
Chinese ambassador to Thailand Guan Mu said on Friday that G-to-G rice sales had been a common practice between the two countries for several decades, but that this was no longer the case. The Chinese government no longer purchased rice from Thailand directly, but instead encouraged private firms to do so, he said.
China was recently asked to buy rice from Thailand to help boost the incomes of Thai farmers, he said. China supported the idea and reached the G-to-G rice agreement in principle with Thailand.
But the deal was strictly in principle. In practice, the Chinese government had only assigned private companies to discuss rice purchases directly with Thai exporters, the ambassador said.
Mr Guan said he was aware that deals had been signed for Chinese companies to purchase hundreds of thousands of tonnes of rice. But he rejected the Thai government's claim that five million tonnes of rice had been sold.
He said Beijing does not buy rice exclusively from Thailand and does not buy such large quantities because China is mostly self-sufficient when it comes to rice. It purchases rice from Thailand only to meet the demands of consumers who prefer Thai rice, he said. China also buys rice from Vietnam, Russia and other countries, he added.
But Mr Guan brushed aside allegations that the G-to-G rice agreements between Thailand and China had been manipulated for the purposes of corruption.
He said the Chinese government is not the buyer, but had played a significant role in convincing companies and state enterprises to buy rice from Thailand.
He said the Chinese government knows nothing about the opening of letters of credit concerning the deals. The matter rests with the private companies that are engaging directly with Thailand, he said.
Mr Ong-art claimed that efforts had been made to tamper with documents from a private Thai firm which had sold rice to China in a bid pass them off as G-to-G contracts.
He said the G-to-G agreements have now become "government-to-gang" deals.
He said the money to buy rice under the agreements had not been transferred by the Chinese government. Rather, he said, the money had originated in Thailand and was transferred through a Thai bank by a rice dealer who had re-circulated rice to benefit from the pledging scheme.
Democrat leader Abhisit Vejjajiva urged the public to monitor the rice-pledging scheme, which he insists is damaging the country's economy.
Mr Abhisit said the country would lose 200 billion baht a year through corruption if the government persists with the controversial scheme.
Government spokesman Tossaporn Serirak and deputy secretary-general of the prime minister Suranand Vejjajiva declined to comment on the Chinese ambassador's statements, saying they would first need to seek clarification of what was said, as the issue is sensitive and could affect relations between the two countries.
Commerce Minister Boonsong Teriyapirom, who oversees the rice scheme, could not be reached for comment.
During the censure debate last week, Democrat MP for Phitsanulok, Warong Dechgitvigrom, alleged that GSSG Import & Export Corp, a China-based trading firm, may have acted as a front for a Thai rice exporter seeking to exploit the rice-pledging scheme.