The Democrat Party plans to seek a ruling from the charter court to consider if the joint parliament meeting on charter amendments is constitutional.
The party said the joint meeting which passed three charter amendment bills at the first reading on Wednesday night might be unconstitutional and could have violated meeting regulations.
Democrat MP for Phatthalung, Nipit Intarasombat said the meeting was short of a quorum.
The meeting could also violate the constitutional provisions regarding parliamentary meetings, and the ongoing charter change process might be rendered invalid.
He said the three committees set up to scrutinise the charter change bills must send the bills back to parliament and a new joint sitting should be called to reconsider them.
Democrat MP for Bangkok Wirat Kalayasiri said the party will seek a ruling from the Constitution Court on the constitutionality of the joint meeting.
On Wednesday, after parliament passed the first reading of the three charter bills, the meeting was short of a quorum as lawmakers discussed the number of days needed for the three scrutiny committees, each comprising 45 members, to complete the job.
The government MPs proposed the scrutiny be finished in 15 days, while the opposition said the scrutiny process should take 60 days to complete.
At the time, a quorum check was called.
Only 295 lawmakers were present, short of the required 323.
Democrat MP for Phitsanulok Warong Dejkitvikrom said that instead of adjourning the meeting, Parliament president Somsak Kiatsuranont allowed the meeting to proceed and decided the period for the scrutiny of the three bills should be 15 days.
Mr Somsak also instructed the three scrutiny committees to hold their first meetings yesterday at 11am.
Opposition chief whip and Democrat list MP Jurin Laksanavisit said the charter amendment process had a hidden agenda, to serve the political and economic interests of certain groups of people.
Mr Jurin said what happened might have made the charter amendment process unconstitutional, as did Mr Somsak's action.
The opposition would look into the relevant laws and take action, Mr Jurin said.
At the meeting of the three charter scrutiny committees yesterday, Senator Krit Arthitkaew, of Kamphaeng Phet, was appointed chairman of the committee to scrutinise the bill to amend Section 190.
Samart Kaewmeechai, a Pheu Thai MP, was appointed chairman of the committee to scrutinise the bill to amend sections concerning senators' terms and whether they should be appointed or elected.