Senator Premsak Piayura is expected to submit a motion to parliament on Thursday seeking a Constitutional Court ruling on whether parliament has the authority to amend Section 256 of the legal code to pave the way for setting up a charter-drafting assembly (CDA).
The motion has already secured the support of 40 senators, as required by the law, some reports say.
It is likely to be submitted to Parliament President Wan Muhamad Noor Matha on Thursday to face a vote, as parliament begins its first reading of charter amendment bills, including those proposing changes to this section.
Earlier, Dr Premsak told the media he would hold a press conference on the planned motion, but he failed to show up.
The senator could not be reached for comment and the Senate Secretariat later told the media he would not be giving any interviews.
The motion is believed to seek clarification on whether deliberation of the bills proposing wholesale charter changes can begin without a referendum first being held.
A 2021 Constitutional Court ruling requires this before and after any move to rewrite the constitution.
As a result, obervers say, any vote without public approval could be deemed unconstitutional.
Senator Pisit Apiwattanawong said on Wednesday that senators have not discussed the charter amendment proposals and the Senate whips have no authority to control how senators decide.
He said Dr Premsak's motion was also not discussed and he had no idea if the motion had been submitted yet.
Mr Pisit said the Senate has been allocated six hours for the charter amendment debate, but the number of participants has not yet been finalised.
Pheu Thai deputy leader Chousak Sirinil expressed concern about a possible walkout in a move to suspend the deliberation of the bills seeking to amend Section 256.
"I'm concerned they will argue over if the meeting is unconstitutional and some might stage a walkout or propose that a fresh ruling be sought before the debate can process," he said.
Mr Chousak, who also serves as Prime Minister's office minister, said the deliberation process should proceed or the court may refuse to consider the case on the grounds there are no parliamentary proceedings underway.
Mr Wan insisted on Monday the deliberation of the bills seeking wholesale changes to the constitution would proceed as planned.
