Opposition whips are to discuss what action to take against the 312 MPs and senators who proposed the "unconstitutional" charter amendment to change the composition of the Senate.
Opposition chief whip Jurin Laksanavisit said on Thursday that the discussions would be held at a meeting next week.
The Constitution Court said on Wednesday that the proposed amendment was unconstitutional. Judges ruled by a vote of 5-4 that the amendment to make the Senate a fully elected chamber violates Section 68. The court also voted 6-3 that the process adopted to pass the amendment was improper and violated Section 291.
The 312 MPs and senators who backed the amendment could face petitions that they be impeached and criminal lawsuits under the Criminal Code, since the charter amendment draft has already been forwarded to His Majesty the King for endorsement and could arguably be considered a false document, Mr Jurin said.
United Front for Democracy against Dictatorship (UDD) core member Jatuporn Prompan has called on the Pheu Thai Party to push for a third reading vote in parliament to amend Section 291 of the charter, which would allow the set-up of a charter drafting committee to rewrite the whole constitution.
Mr Jurin claimed the push to allow the set-up of a charter drafting committee is ultimately intended to make it possible to clear those implicated in corruption cases.
He said UDD supporters really want to amend Section 309 of the constitution to clear those who faced legal action as recommended by the now disbanded assets scrutiny committee set up by the 2006 coupmakers.
Chief government whip Amnuay Khlangpha said whips would have to go through the court's ruling before meeting with the 312 lawmakers who backed it to decide what to do next.
He insisted that all members of parliament have the right to propose charter amendments.
On Mr Jatuporn's suggestion that government vote on the third reading of the Section 291 amendment draft, Mr Amnuay said the government whips were yet to discuss the matter.