The Constitutional Court on Wednesday rejected a petition lodged by the Ombudsman on the incomplete oath of office recited by Prime Minister Prayut Chan-o-cha in July.
The court resolved unanimously that it was "not in its authority" to make a ruling on the oath-taking during the cabinet swearing-in ceremony before His Majesty the King.
The petition was lodged through the Office of the Ombudsman on Aug 20 by Panupong Churak, a Ramkhamhaeng University student, who cited Section 213 of the charter on the right of citizens to complain if they consider their constitutional rights are violated.
Mr Panupong claimed the incomplete oath could invalidate the cabinet's formation and its statement and implementation of policy, affecting the government's ability to work for the people's benefit.
On Jul 16, Gen Prayut led cabinet ministers at the swearing-in ceremony before HM the King at the Ambara Villa in Dusit Palace.
However, the prime minister failed to recite the final sentence of Section 161 of the constitution, which requires the oath-taker to uphold and abide by the constitution.
The omission prompted questions over the validity of his premiership among opposition parties and political activists.
However, the court said "the oath-taking concerns an action which reflects a 'specific relationship' between the cabinet and the King and is considered a political issue under an act of government, stated in Section 47 of the organic law on Constitutional Court procedure.
"The court thus cannot accept the petition for consideration," it said.
The court went on to explain that after Gen Prayut and his cabinet completed the oath-taking, His Majesty the King responded by offering his moral support to the government in carrying out its duties in line with the oath it had made.
Then, on Aug 27, amid growing criticism directed at Gen Prayut for failing to complete the oath, His Majesty the King issued a written message of support to the prime minister and cabinet, inviting them to perform their duties in line with their oaths of office.
Gen Prayut and cabinet members received the message in turn before a portrait of the King at Government House.
The message was the speech the King had delivered to them during the swearing-in ceremony at Ambara Villa.
The court on Wednesday also dropped a similar complaint over an alleged mistake the premier made during the oath-taking.
The rejected petition was filed by Ruangkrai Leekitwattana, a former member of the dissolved Thai Raksa Chart Party.
Mr Ruangkrai had accused Gen Prayut of violating the constitution, an act tantamount to infringing upon the constitutional monarchy, by failing to identify financial sources to support the government's proposed policies, as required by the charter's Section 162, during a parliamentary session.
The court said it found no evidence to back Mr Ruangkrai's suspicion.
Opposition chief whip Suthin Khlangsaeng said the court's rejection of the petition had not settled doubts, so parliament was still bound to go ahead with its planned debate on the oath-taking.
Mr Suthin, Pheu Thai MP for Maha Sarakham, said the dismissal of the petition also relieved the opposition of concerns it would not be able to perform its duty fully during the debate.
Opposition parties had feared the court review would limit what could be said in parliament, he said.
"But now that the court has dismissed the petition, the debate can proceed without restrictions. We'll be able to scrutinise the matter, and the prime minister now has no excuse to avoid [answering the questions]," he said.
Mr Suthin also said the prime minister would have a more difficult time defending himself and would have to take political responsibility over the oath.
The opposition chief whip said the issue underlined the need to amend the constitution so that a political act deemed unconstitutional is examined by an independent agency.
However, Jade Donavanik, former adviser to the Constitution Drafting Committee, warned on Wednesday the debate may violate Section 112 of the Criminal Code on lese majeste, as the court had ruled the issue was between the King and the cabinet.