Activist Srisuwan Janya on Friday asked the Office of the Ombudsman and the Election Commission (EC) to seek a judicial review of the eight-year limit on Gen Prayut Chan-o-cha's premiership.
The 2017 constitution caps a premier's term limit at eight years, but people are divided as to when his premiership began.
In his petition, Mr Srisuwan asked the ombudsman office and the poll agency to seek the Constitutional Court's interpretation of Gen Prayut's maximum eight-year tenure.
He said the record should be set straight as there are three potential dates regarding when his tenure should expire.
Some believe it should be this month because the count started in 2014 when he was made the prime minister after the 2014 coup.
Another group claims his term should end in 2027 because he was appointed prime minister under the 2017 charter in June 2019. As a result, his eight-year limit would end in 2027.
According to a different way of calculating it, the count started in April 2017 when the current charter was promulgated, meaning Gen Prayut's tenure would end in 2025.
Mr Srisuwan said the Office of the Ombudsman has the authority under Section 231(1) and Section 230 of the charter to ask the Constitutional Court to rule on the matter, while the EC is authorised to do so under Section 170 of the constitution.
Pheu Thai leader Cholnan Srikaew said the opposition camp plans to forward a petition to the House Speaker for submission to the court in the middle of this month.
Meanwhile, the prime minister brushed aside reporters' questions as to whether he was concerned about the issue. "[You should] ask the Constitutional Court," he said.
Earlier, a House of Representatives legal team claimed Gen Prayut's term technically began on June 9, 2019.