The government's position over the Koh Kut island dispute between Thailand and Cambodia has been subjected to a smear campaign, said Noppadon Pattama, a Pheu Thai Party list MP.
He warned against anyone spreading false information claiming Thailand had lost the island in Trat under a memorandum of understanding signed with Cambodia in 2001.
"Koh Kut still belongs to Thailand. That's a fact. No one has ceded the island to Cambodia. I have never heard Cambodia assert territorial claims over Koh Kut," said Mr Noppadon, a former foreign minister.
"Stop spreading lies that Thailand has lost the island. The government is duty-bound to protect Thai territory, and no one is allowed to cede any land."
He said anyone who falsely accuses Pheu Thai of ceding Koh Kut or any territory would face legal action.
He said the 2001 MoU, signed by then-foreign minister Surakiart Sathirathai, served as a framework for negotiations over maritime areas in the Gulf of Thailand claimed by Thailand and Cambodia, as well as matters related to joint development. The MoU provides a framework for diplomatic negotiations based on international law.
Most importantly, the MoU does not affect maritime claims by both countries, as they will continue to retain their respective rights if negotiations fail, Mr Noppadon said.
He further said that negotiations under the MoU must be handled only by the Thai-Cambodian Joint Technical Committee (JTC), which includes representatives from the Department of Treaties and Legal Affairs, the Energy Ministry, and the military.
He said no other parties are now allowed to join the talks, adding that the prime minister is not involved and that any conclusions from the negotiations must be presented to parliament for review first.
"The government cannot reach any secret deals with Cambodia," Mr Noppadon said.
He said that the misinformation about Thailand losing Koh Kut to Cambodia is reminiscent of when he was falsely accused of handing Preah Vihear Temple to Cambodia when he served as foreign minister.
He said that the 1962 International Court of Justice ruling awarded the temple complex to Cambodia when Field Marshal Sarit Thanarat was prime minister.
"The smear campaign suggesting that any of Koh Kut was ceded to Cambodia is politically motivated with an aim to destabilise the government. All Thais, regardless of their political stance, love their country," he said.
Koh Kut was thrust into the spotlight when key figures of the Palang Pracharath Party (PPRP) held a press conference on Wednesday to oppose any discussions on joint development of oil and gas resources in the Gulf under the 2001 MoU framework, as it could mean Thailand losing some of its territory.
They said the PPRP would submit an open letter to Prime Minister Paetongtarn Shinawatra asking her to halt the talks. The MoU was signed during her father's government, former prime minister Thaksin Shinawatra.
Thirachai Phuvanatnaranubala, a former finance minister and PPRP chairman of an academic committee, said the MoU's contents include a territorial claim by Cambodia, which is against the 1907 French-Siamese treaty, making the entire MoU illegal.