Senators are divided over a proposal seeking to amend the constitution in what is seen as a move to help Prime Minister Prayut Chan-o-cha cling to power.
On Friday, Sen Seree Suwanpanont, chairman of the Senate committee on political development and public participation, admitted the panel had asked the King Prajadhipok's Institute to study the possibility of amending Section 158 of the constitution.
The section stipulates that an individual can only serve as PM for a maximum of eight years, regardless of whether the four-year terms were served consecutively or not.
Mr Seree said the panel concluded that the section should be revised to remove the eight-year limit on a PM's tenure.
Sen Kittisak Rattanawaraha also said he supports the proposal, saying the eight-year limit should be removed.
The ruling Palang Pracharath Party (PPRP) and the main opposition Pheu Thai Party slammed the proposal, saying it is designed to extend Gen Prayut's tenure beyond 2025.
If he were re-elected, Gen Prayut would only be eligible to remain in office until 2025, according to a ruling by the Constitutional Court in September concerning his eight-year tenure limit as premier.
Sen Direkrit Janekrongtham, who serves as a deputy chairman of the committee, said on Saturday the panel had not discussed the proposal yet, and it may be only the personal opinions of some senators.
"The proposal may have been discussed informally among the two senators [mentioned before]. They already have their stance against the limit on the eight-year tenure. The proposal has not been discussed officially yet," Mr Direkrit said.
"They can amend the constitution by raising the matter in parliament. But the opinions of the senators do not represent the collective opinion of the Senate as a whole," he said.
He said amending the charter is not easy as a charter amendment proposal requires the support of at least one-third of senators and at least 20% of opposition MPs.
Since the constitution was endorsed by voters at the 2016 referendum, another referendum is also required to endorse any amendments, he said.
On Friday, Gen Prayut declined to comment on the issue, saying it has nothing to do with him.
According to a source, senators who support Gen Prayut are seeking to amend the charter to extend his tenure further as the tenure of the coup-appointed Senate will expire next year.
Gen Prayut has the support of about 180 senators, the source said.
The outcome of the coming election will also be factored in. The charter amendment bid will go ahead if Gen Prayut's United Thai Nation Party wins enough House seats to join a coalition government after the next poll, the source said.
Sen Wanchai Sornsiri on Saturday took to Facebook to downplay a report about the move seeking to amend the constitution to help Gen Prayut stay on in power.
"The report about senators amending the charter to help Gen Prayut is intended to discredit the Senate. It is political nitpicking," he wrote.
"Senators have already concluded that any party that wins a majority of House seats should lead the government and senators should support the person [nominated as a PM candidate] in line with the people's mandate," he added.
Chumsai Sriyaphai, deputy secretary-general of Pheu Thai, on Saturday slammed the proposal, saying that Section 272 of the charter should be revised to remove the Senate's power to co-elect a PM with MPs instead.
Paiboon Nititawan, deputy leader of the PPRP, said the charter amendment proposal will face resistance as it is intended to serve an individual, referring to Gen Prayut.
"It will do more harm than good for Gen Prayut and could lead to a political crisis," he said.