
The Cabinet failed to seek a green light from Parliament when it approved the communique, signed by Foreign Minister Noppadon Pattama and Cambodian Deputy Premier Sok An, in support of that country's bid to have the temple ruins listed as a World Heritage site.
The government has argued it did not regard the communique as an international agreement.
Article 190 of the Constitution stipulates that any treaty providing for changes in Thai territory or extraterritorial areas over which Thailand has sovereign rights must be approved by the National Assembly.
Following the court's ruling, a group of senators held a press conference at Parliament to call on the foreign minister to resign immediately, in order to take responsibility for his actions. They said they would file a case against the entire Cabinet via the National Counter Corruption Commission (NCCC) with the Supreme Court's Criminal Division for Political Office Holders.
The group's leader, Senator Kamnoon Sidhisamarn, said Noppadon should take responsibility for failing to follow the Constitution. "It's a matter of conscience and etiquette for the minister to resign immediately," he said. Kamnoon said in addition to Article 190, Cabinet members had also violated certain provisions of the Penal Code.
"We'll see whether Articles 119 and 120 of the Penal Code have been violated. The maximum penalty is capital punishment," he said.
Kamnoon, who was among 77 senators seeking the Constitution Court's ruling on the status of the communique, rejected criticism that senators were trying to corner the government.
"This is a big issue," he said. "It's not just about Preah Vihear Temple. It's about ministers who go against the Constitution and fail to obey the law."
Senator Rosana Tositrakul called on the foreign minister to resign before he faced an impeachment campaign. Rosana said the NCCC might begin an immediate investigation against the Cabinet if it found that provisions of the Penal Code on national security had been violated.
Senator Somchai Swangkarn also urged the foreign minister to resign.
"What Noppadon must do is take responsibility by resigning. If he is worried there will be an angry crowd at the airport, he may opt to land in Phnom Penh instead," he said.