The six opposition parties have agreed to submit a motion to Parliament to amend the constitution, the first step that would open the door for a rewrite of the charter by a new council.
Sutin Klungsang, who led the opposition whips, said on Saturday more than 100, the required number of MPs, had signed the motion to amend Section 256, which governs how the 2017 constitution can be amended.
The motion will be sent to the House speaker tomorrow. After that, coalition parties, the civil and public sectors, as well as other political parties, may submit their own versions of the bill, he said.
The Pheu Thai MP for Maha Sarakham said he hopes all versions of the bill would be deliberated in the current meeting session.
Pol Col Tawee Sodsong, secretary-general of the opposition Prachachat Party, said amending Section 256 is the first step.
"A section should also be added to ensure people from all walks of life can participate like the Constitution Drafting Council in 1997. After a discussion, we agree we should allow people aged 18 or older to take part," he said.
The consensus among opposition parties is the 2007 charter is undemocratic as clearly seen in the last section, which legalises all National Council for Peace and Order (NCPO) announcements and orders.
On NCPO-appointed senators being able to vote for a prime minister, Pol Col Tawee said opinion is split.
"Some believe the senators should no longer be allowed to elect a PM while others want to abolish appointed senators altogether. We haven't reached an agreement on this," he said.
On the sensitive issues of the monarchy and national security, he said there was discussions about these but insisted the country must remain a democracy with the king as head of a single state.
Section 256 outlines the complicated process of changing the charter, while Section 255 prohibits changing the rule of the country from a democracy with the king as head of state and the type of state from a single state.
Changes involving six chapters, including those on the King, the qualifications of certain positions, courts or independent organisations, require a referendum.
One-tenth of MPs or senators may ask the Constitutional Court to rule whether an amendment involving the two untouchable issues requires a referendum before it can be submitted to the King.