Prayuth led the inauguration of his Cabinet in a ceremony presided over by the king on July 16.

However, he omitted a phrase in the oath of office in which he was supposed to pledge to uphold every aspect of the constitution. The omission has raised questions over whether the inauguration was legally valid.

Prayuth told reporters Friday that he was continuing to conduct his duties "to the best of my abilities because I am the prime minister."

The oath of office is required under Article 161 of Thailand's Constitution, which includes the complete oath and states it must be said to the king before Cabinet ministers take office.

Prayuth's failure to recite the oath in full, which also led to other ministers making the same error because they repeated what he said, was pointed out by opposition politician Piyabutr Saengkanokkul during a Parliament session on July 25.

Legal activist Srisuwan Janya filed a complaint over the issue to the Office of the Ombudsman on Monday which has been accepted for consideration.

Prayuth led a military junta that seized power in 2014 and was dissolved with the inauguration of the new Cabinet. The junta had ruled with a heavy fist and regularly cracked down on its critics. It also introduced new election laws to favor Prayuth's return as prime minister.

Mongkolkit Suksintaranont, a leader of a political party that was part of Prayuth's coalition, said on Thursday that he and four other parties which hold single seats in the House of Representatives were leaving the coalition.

"I did not think that being part of the government coalition would mean that we would have such little freedom," Mongkolkit said, adding that he had been told to refrain from criticizing the government in Parliament sessions.

When asked how he would handle the issue of the Cabinet's incomplete oath of office, Mongkolkit said, "If I was prime minister, I would have resigned already."