The Constitution Drafting Committee (CDC) and the National Legislative Assembly (NLA) have concluded in principle that candidates for the prime minister's post must be nominated by political parties first.
However, they have agreed to maintain a provision under the draft constitution which gives a joint meeting of MPs and the Senate the power to suspend the rule relating to the prime ministerial candidates.
This would allow for an "outsider" to be nominated, a move which critics fear could ignite a new round of political conflict.
NLA deputy chairman Surachai Liangboonlertchai met Monday with CDC chairman Meechai Ruchupan to discuss revisions to the draft charter in line with the extra question relating to the Senate's power to help MPs select a prime minister.
The question was approved along with the draft constitution in the Aug 7 referendum.
The CDC is required to change the draft in line with the question, which was proposed by the NLA.
Voters were asked whether the 250 appointed senators serving a five-year term during the transition period to full democracy should be allowed to join the House of Representatives in the voting process to select a prime minister for this period.
However, a clause in the draft requires parties to submit up to three prime ministerial candidates each before the election, with the House of Representatives then choosing the prime minister from the lists after the poll.
But if the House cannot choose the prime minister for whatever reason, it can then propose a motion to convene a joint House-Senate session to consider whether to ignore the three-candidate rule as stipulated by Section 272 of the draft charter.
Even if both houses agree to suspend the three-candidate rule, the process of nominating and voting for a prime minister outside the parties' lists will remain exclusively in the hands of the House.
At this stage, the Senate will have no role in choosing the prime minister.
But since the extra question has now been endorsed at the referendum, the draft charter must be amended to allow the Senate to help MPs choose a prime minister.
Mr Surachai said the change would authorise the joint meeting of the Senate and MPs to pick a prime minister, and the CDC will consider how to revise the draft charter so this would not affect the draft charter as a whole.
Mr Surachai said Section 272 of the draft charter needs to be retained to prevent any future political problems, and the section may be amended to accommodate the extra question.
After the meeting, Mr Meechai said revision to the draft charter should be finished within 30 days, adding representatives of the CDC and the NLA will meet again to discuss the change on Friday.
Mr Meechai also stood by a draft charter provision that gives MPs, not the Senate, the power to initiate a motion to convene a joint House-Senate session to consider whether to ignore the three-candidate rule.
He also said the CDC intends for a prime minister to come from the lists of candidates submitted by political parties first.
It would be good if the NLA explained this to the public and this means the CDC and the NLA shared the same understanding, Mr Meechai said.
When revisions to the draft charter are complete, the CDC will submit the revised draft to the Constitutional Court for consideration within 30 days.
If the court sends the draft back to the CDC for more revisions, the CDC must make the changes within 15 days before the draft can be enacted, Mr Meechai said.
A legal question has been raised as the draft charter says the Senate can only vote on whether to accept the prime ministerial candidate nominated by the parties.
However, questions remain as to what to do if the party-nominated candidates repeatedly fail to win half the votes in parliament.
