Prime Minister Prayut Chan-o-cha reprimanded Thai Civilised Party leader Mongkonkit Suksintharanon for taking an explosive into parliament to test the security system which he alleged was "inadequate".
The premier did not give weight to Mr Mongkonkit’s claim but instead questioned his behaviour following opposition raised by House Speaker Chuan Leekpai.
"Everyone, no matter who you are, must think carefully before acting," Gen Prayut said on Thursday.
"Their actions can cause [unwanted] impact on other things."
The prime minister said he will leave the Mongkonkit case with the authorities to determine what his real motive was.
Mr Chuan added he had assigned legal experts to investigate Mr Mongkonkit's behaviour, which will be judged according to law.
"Parliament must show people it can be a model for keeping in line with the law," he said.
Mr Mongkonkit had claimed earlier that a weapon detector installed in parliament had failed to detect the explosive trinitrotoluene, or TNT, which was carried by a police officer on Wednesday on his direction. Despite finding what he labelled as a flaw, Mr Mongkonkit admitted on Thursday his actions were not right.
"I would like to apologise to Mr Chuan for not asking for permission to conduct the test," Mr Mongkonkit said.
He said he is ready to receive whatever punishment seen fit by Mr Chuan, whom he treats as his "father".
Mr Mongkonkit’s contentious act took place during a press conference on security issues held in parliament by the House committee on military affairs, for which Mr Mongkonkit is a spokesman.
The committee discussed security issues after a US company gave the government and the Royal Thai Police two bomb detectors to use during the Asean summit this weekend.
Mr Mongkonkit took the opportunity to test the detectors in parliament by asking an officer from the explosive ordinance disposal unit to take TNT into his room, which is located beneath the House auditorium.
The failure of the detectors to pick up the explosives meant that an ill-minded person could use a bomb to attack parliament, Mr Mongkonkit said.
His description of the two US-made detectors — which were able to detect TNT bombs during a test in the insurgency-torn provinces in the far South — drew criticism that he might have been given some benefits from the company.
"That’s not true," Mr Mongkonkit said.
"I don’t know the company employees personally. I just met its officials for the first time yesterday [Wednesday]."