Former prime minister Thaksin Shinawatra's personal interests in energy production lie behind government attempts to change the charter, the Democrats say.
The government was attempting to alter Section 190 of the constitution to further Thaksin's interests in energy production in a disputed area in the Gulf of Thailand, the Democrats told the joint sitting yesterday.
A joint sitting of parliament yesterday debated the second reading of a government draft to amend Section 190 of the constitution.
The section requires the government to seek parliamentary approval before sealing any international agreements that significantly affect territory, sovereign rights, national economic or social security, or trade and investment deals with binding clauses.
The government wants to change Section 190 so it covers only international agreements that affect Thai territory or involve free trade and investment.
The opposition's concern centres on the area claimed by both Thailand and Cambodia in the Gulf of Thailand.
The World Bank estimated there were 2 billion barrels of oil and 10 trillion cubic feet of natural gas worth 5 trillion baht in the 26,000 sq km disputed area, Democrat MP Sansern Samalapa told parliament.
"Amending Section 190 will give too much authority to the cabinet in dealing with disputed areas," he said. "I fear that in the future some businessman can convince the government to put itself at a disadvantage against Cambodia to bargain for personal business interests.
"This will be a conflict of interest and will cheat the nation because the energy business will have been conducted for the benefits of this person and his family."
The Democrat also quoted Gen Tea Banh, deputy prime minister and defence minister of Cambodia, as saying Thaksin was interested in the energy business.
The remark drew protests from Pheu Thai Party MPs. Parliament President Somsak Kiatsuranon then tried to end the debate. However, Senator Khamnoon Sitthisamarn argued at least 23 senators had doubts about the amendment draft and should have their say.