The opposition is gearing up to grill the government in a general debate tentatively planned for shortly after the New Year as it zeroes in on what it describes as the unethical appointment of the prime minister's new secretary-general.
Pheu Thai Party leader, Cholnan Srikaew, said on Wednesday the opposition was working on the motion and close to wrapping it up.
The bloc expects it will examine the proposal to hold a general debate for the last time on Dec 26 before submitting it to the parliament president no later than Dec 28.
He said the debate could get underway around the third or final week of January.
Dr Cholnan, however, stopped short of spelling out which issues would be raised.
"All I ask of Prime Minister Prayut Chan-o-cha is that he refrain from dissolving the House in order to avoid the debate. It would be a shameful way to try and escape scrutiny," said Dr Cholnan, who is also the opposition leader.
He made the remarks after Gen Prayut appointed Pirapan Salirathavibhaga, leader of the newly established United Thai Nation (UTN) Party, as the prime minister's secretary-general.
Critics say Mr Pirapan, in his new post, wields considerable power. In some cases, he could act on the premier's behalf, thus serving as a de facto "No.2" prime minister.
The opposition noted that while it was within the law for the prime minister to name a secretary-general, the latest appointment may not ethical, given the potential political advantage to be gained from the post in the build-up to the next general election, tentatively scheduled for May.
This poses a concern as Mr Pirapan leads the UTN, a party Gen Prayut has been speculated to join and which is likely to nominate him as its prime ministerial candidate in the next poll.
In addition, critics contend the previous secretary-general, Dissathat Hotrakit, was fully competent in the role and did nothing to warrant being shifted to a lower role as one of the PM's advisers.
Dr Cholnan said he believed Mr Pirapan's appointment was not work-related or based on merit.
"Is this by any chance beneficial to a certain party that Gen Prayut is reported to be moving to?" the Pheu Thai leader asked rhetorically.
He said the opposition was looking into whether the appointment of Mr Pirapan was politically motivated or amounted to an attempt by a political party to unfairly gain influence, which would be illegal.
Somkid Chuakong, another Pheu Thai MP for Ubon Ratchathani, said a prime minister secretary-general could sometimes give approvals on Gen Prayut's behalf. Mr Pirapan's appointment "may constitute an abuse of power", he added.
The Election Commission should investigate the matter, he said. Mr Somkid reasoned that Gen Prayut is known to be connected to the ruling Palang Pracharath Party, which nominated him as premier in the previous poll.
Gen Prayut's decision to pick someone from another party to be his secretary-general could be construed as political interference, he said.
Chaiya Promma, another Pheu Thai MP, said that as chair of the House committee following up on budget spending, he would be closely monitoring how Gen Prayut disburses the funds he is entitled to use ahead of the election.