The Constitutional Court has agreed the draft charter needs amending for senators to join MPs in a vote to suspend the rule which will enable candidates for the premiership to come from outside political party lists.
The court also decided the tenure of the next parliament will depend on the lifespan of the next Senate.
The judges decided the Constitution Drafting Committee (CDC) must proceed to rectify two areas of the draft charter, in light of the extra question in the referendum approved on Aug 7.
The extra question asked if the Senate should join MPs in selecting the next prime minister.
The CDC will amend the draft charter in compliance with the ruling.
The CDC, the court ruled, must review the part in the draft charter where it says the House of Representatives will vote to suspend the rule requiring prime ministerial candidates to come from political party lists during the first five years after the new constitution is enacted.
The committee must make a change so both houses vote on the suspension.
After the rule is suspended, at least 1/10 of MPs can initiate a motion to propose individuals not included on party lists to be prime ministerial candidates.
However, the court said senators are not permitted to nominate a prime minister, arguing the charter is designed to place the process of nominating and voting for the prime minister in the hands of MPs, so the selection of a prime minister is off-limits to the Senate.
Meanwhile, CDC chairman Meechai Ruchupan said yesterday the CDC is expected to revise the draft and come up with a final draft within 15 days.
He said a panel has received a judicial review of the draft from the court and is considering the changes.
He said the final draft will be submitted to Prime Minister Prayut Chan-o-cha who will forward it to His Majesty the King for endorsement.