House Speaker Somsak Kiatsuranont said on Monday he would not appear before the Constitution Court to explain the charter amendment bill seeking to amend Section 68 of the constitution.
Mr Somsak is one of the 312 MPs and senators who proposed the bill.
Senator Somchai Sawaengkarn filed a petition asking the Constitution Court to rule whether the lawmakers had violated the constitution.
The court accepted it for consideration and sent a letter to the 312 lawmakers asking them to appear before it to give their explanation.
The Pheu Thai Party passed a resolution that its MPs should defy the court's order. It also plans to issue an open letter challenging the Constitution Court's power.
Mr Somsak said that as speaker of the House of Representatives he must be impartial and could not comply with the party's resolution. But as an MP he agreed with the party and in this capacity would not appear to offer his explanation to the court.
He said he had also signed the party's open letter.
Since the court had earlier ruled that amending the charter must be made by sections, he did not see it necessary for him to explain to the court again, he said.
Mr Somsak said neither former prime minister Thaksin Shinawatra nor the Pheu Thai Party had given him any instruction concerning this matter.