The opposition will seek a Constitutional Court ruling over the dispute regarding the eight-year limit on Gen Prayut Chan-o-cha's premiership next week, with opinions from former charter drafters to be included in a planned petition.
Natthawut Buaprathum, a Move Forward Party deputy leader, said opposition parties are working on a petition to be submitted to the court on Wednesday.
He said if the petition has grounds, the court is expected to accept it and could issue a temporary injunction suspending Gen Prayut from duty.
"The opposition will ask the court to [suspend Gen Prayut] because we don't want to see any adverse impact on the running of national affairs and budget spending [if Gen Prayut continues to carry out his duties]," he said.
He said the opposition will include the minutes of a Sept 7, 2018 meeting attended by a charter drafting committee to support the view that Gen Prayut's maximum eight-year tenure ends later this month.
According to an excerpt from the document, Meechai Ruchupan, chairman of the 2017 charter drafting committee, said the eight-year tenure of a PM in office before the charter should span both the time before it came into effect and the period after, if the same premier served again after the new constitution was put in place.
"The dispute over Gen Prayut's eight-year tenure may not be laid to rest this month," Mr Natthawut said. "A time bomb may explode if Gen Prayut refuses to step down in a timely manner."
Deputy Prime Minister and Bhumjaithai Party leader Anutin Charnvirakul, part of the government coalition, said he is not concerned about when Gen Prayut's tenure will conclude.
"We will do all we can to support the prime minister to steer the government until the end of its tenure," he said. "The legal interpretation of the matter rests with the Constitutional Court."
"In the future, if the prime minister wants to campaign for the next general election, the party he belongs to will be our rival," Mr Anutin, who also serves as public health minister, said. "We will just follow the rules of the game."
Section 158 of the constitution limits the term of a PM to eight years, but politicians are divided on when Gen Prayut's eight-year tenure concludes, with three potential dates that have been bandied around by various groups.
Some believe it should be this month because the count started in 2014 when Gen Prayut first assumed the role of PM after the coup that year.
Another group claims his term should end in 2027 because he was appointed PM under the 2017 charter in June 2019. As a result, his eight-year limit would end in 2027.
According to another metric, the count started in April 2017 when the current charter was promulgated, meaning Gen Prayut's tenure would end in 2025.
Previously, Deputy Prime Minister and Palang Pracharath Party (PPRP) leader Prawit Wongsuwon expressed support for Gen Prayut to survive the legal dispute over the tenure and carry on for two more years.
Observers said that if Gen Prawit is proved correct, then it would mean that Gen Prayut would not have to step down until at least 2025.
If the general election is held next year and the PPRP wins a majority of House seats and forms a government again, Gen Prayut is expected to return as premier.
While the new government has a four-year term, Gen Prayut could only serve as PM for only two years as his premiership would end in 2025 according to the interpretation mentioned above, observers said.
After that, parliament would have to choose a new PM to serve out the remaining two years of the government's tenure.
Wanwichit Boonprong, a political science lecturer at Rangsit University, said Gen Prawit's remarks show an attempt to strike a compromise between his supporters and Gen Prayut's backers within the ruling party.
In addition to Gen Prayut, the PPRP may also nominate Gen Prawit as another PM candidate, and he could succeed Gen Prayut in 2025, Mr Wanwichit said.
Such a move would also convince current coalition parties, such as Bhumjaithai, to maintain their alliance with the PPRP, though the ruling party has a higher price to pay, such as offering key cabinet portfolios in exchange for support, he said.
Yutthaporn Isarachai, a political scientist from Sukhothai Thammathirat Open University, also agreed Gen Prawit could also emerge as another PM candidate in the next election.
Mr Anutin will also stand a good chance of succeeding Gen Prayut as PM, depending on the number of House seats Bhumjaithai will win in the next poll, he said.
"There is a possibility the current coalition parties will strike a deal to maintain their alliance and form a coalition government again after the next poll.
"We might even see Gen Prayut and Mr Anutin taking turns serving as prime minister for two years each," he said.