Prime Minister Prayut Chan-o-cha has warned that the ongoing charter amendment process must leave the royal institution untouched.
Gen Prayut made the remarks as he chaired a mobile cabinet meeting in Krabi, according to sources at the meeting.
During the meeting, he was briefed on Tuesday's parliamentary debate on a charter amendment bill pushing for "a people's constitution". Lawmakers will on Wednesday decide whether to accept the bill in the first reading.
Gen Prayut was quoted by the sources as saying: "The country, the religion and the monarchy must remain [intact]. MPs must not act in a way that affects the royal institution."
The prime minister also stressed the need for government MPs to abide by party resolutions when they vote on the bill, the sources said.
After the cabinet meeting, Gen Prayut held a discussion behind closed doors with Deputy Prime Minister and leader of the ruling Palang Pracharath Party (PPRP) Prawit Wongsuwon and Interior Minister Anupong Paojinda for about an hour, the sources said.
Natural Resources and Environment Minister Varawut Silpa-archa, who chairs the coalition Chartthaipattana Party's policy and strategy committee, said that party MPs will discuss how to vote on the bill before today's vote.
He said many party MPs told him they would reject the bill because the current constitution was endorsed at a referendum on Aug 7, 2016.
Asked about possible unrest if the bill is rejected, Mr Varawut said supporters of the bill should respect the more than 16 million people who voted in support of it at the referendum.
The bill, which has the support of more than 135,000 eligible voters, proposes six key changes to the 2017 Constitution.
They include a proposal seeking to abolish the Senate, which is seen as the proxy for the now-dissolved National Council for Peace and Order. It also seeks to increase the MPs' and opposition's power to scrutinise government actions.
The bill is being sponsored by the Internet Law Reform Dialogue (iLaw), the Re-Solution group, the Progressive Movement and the Move Forward Party.
Digital Economy and Society Minister Chaiwut Thanakamanusorn, who sits on the PPRP's executive board, said the bill contains proposals that are hostile toward the Senate and thus senators are expected to reject it.
He added that the PPRP would hold a meeting today to decide how to proceed with the vote. "We have to listen to all groups in society, not any group in particular," Mr Chaiwut said.
Piyabutr Saengkanokkul, secretary-general of the Progressive Movement, voiced disagreement with the current constitution which allows senators to decide on the passage of charter amendment proposals. The bill requires the support of at least one-third of the Senate, or 84 senators, and at least 20% of opposition MPs, or 43, to pass.
"It is strange that fewer than 100 senators are allowed to block charter amendment proposals," he said, adding senators should make sacrifices by voting to scrap the Senate.
Mr Piyabutr encouraged parliamentarians to vote to accept the bill through to a third and final reading. If it passes the third reading, it will still be put to a referendum, he said.