The National Reform Council (NRC) Monday rejected a proposal by its whip to invite and nominate five outsiders to sit on its 36-member charter drafting panel.
Council members voted 175 to 39 in favour of selecting 20 candidates for the Charter Drafting Assembly (CDA) from its own members only.
Last Wednesday, a meeting chaired by the NRC whip voted 10 to 8 to accept a proposal to give five of the 20 seats to outsiders, including representatives of the political divide.
The decision came under heavy fire by NRC members.
In a debate prior to the vote, NRC members Borwornsak Uwanno and Alongkorn Ponlaboot defended the decision to include outsiders.
They stressed that by including outsiders in the drafting process, the new charter would be safe from criticism.
"The new charter intends to put an end to the problem that has dragged on for 10 years. To prevent the charter being criticised, we must let those involved in the political conflict play a part in its creation," Mr Borwornsak said.
The outsiders in question were expected to be representatives from the People's Democratic Reform Committee, the United Front for Democracy against Dictatorship, and the Democrat, Pheu Thai and Chartthaipattana parties.
By engaging outsiders in the drafting process, the NRC would be able to prove it was not doing favours for any party and was serious about making the process inclusive.
Mr Alongkorn said the move would prevent the new charter from being criticised as the fruit of the military coup.
If that was the case, he said all their efforts would go to waste.
However, a majority of the NRC members was not convinced that engaging outsiders would ease political divisions.
Narong Phutticheewin said it was customary to make the offer, although it would be unlikely to heal any political rifts.
The NRC was also divided yesterday on how it would choose 20 CDA members. Some suggested that 11 people should represent 11 areas of required reforms, four should represent each region and the remaining five should be voted for by council members.
Meanwhile, Prime Minister Prayut Chan-o-cha yesterday insisted the Thailand Reform Institute is not affiliated to the government and would not do its reform work.
The institute is led by Sangsidh Piriyarangsan, a NRC member and former vice-chairman of the National Economic and Social Advisory Council, who set up the civic group at the weekend to work in parallel with the NRC.
However, Gen Prayut said the NRC is the only reform body tied to the government.
The public can send in reform proposals through several outlets, he said.
They include the Internal Security Operations Command, the Damrongtham Centre and Secretariat Office of the NRC.