Srisuwan Janya, secretary general of the Constitution Protection Association, filed a complaint with the Ombudsmen Office against the National Legislative Council's members for voting Prayuth Chan-ocha as prime minister.
He asked Chalermsak Chantaratim, secretary general of the office, to investigate into the NLA members.
That all 194 NLA members voted Gen Prayuth as PM constitutes a conflict of interests, therefore breaching the 2014 interim charter, since the NLA was nominated by the National Council for Peace and Order (NCPO) of which Gen Prayuth is chief.
Mr Srisuwan also claimed the interim charter had checks and balances between the NCPO and the prime minister. It stipulates the NCPO could propose that the NLA vote to remove the PM.
When the NCPO chief and the PM are the same person, there is no way to remove the prime minister if he does anything wrong, Mr Srisuwan said.
This is in violation of the basic principle of constitutional monarchy.
"The Constitution Protection Association therefore asks the Ombudsman to forward the case to the Constitutional Court for ruling whether the NLA's resolution and the acceptance of the PM position by Gen Prayuth is in breach of the interim charter.
"The same controversy also applies to other NCPO members who may be appointed cabinet ministers," he said.
Early this month, Mr Srisuwan also filed a complaint with the Ombudsman Office, claiming the composition of the NLA was not diverse as stipulated in the interim charter.