The Political Development Council (PDC), a body set up after the 2007 coup, has joined the chorus of disapproval of the constitution draft and urged the National Reform Council (NRC) to reject it.
The draft has conflicts of interest and perpetuates the mandate of the powers-that-be, chairman Teerapat Serirangsan said on Saturday.
It also lacks justification for some of its provisions and is based on outdated principles, in his view. Above all, it makes amendments very difficult, said Mr Teerapat, a former Prime Minister's Office minister in the Surayud Chulanont government.
The council also disagrees with spelling out in the charter the establishment of the National Reform Steering Council and the National Reform Strategy Committee.
Other critics have said that these two bodies were simply a way to entrench the positions of the people currently serving in the various bodies that were hand-picked by the military junta.
"Some of us even proposed that [the charter draft] be scrapped," said Mr Teerapat. "We finally agreed we would submit our proposals to the constitution writers first and would decide later if they do not accept them."
Most of the PDC members think a public referendum on the charter is necessary. After all, a constitution is the highest law and a social contract, he said.
"If we are to adopt it, it should be endorsed by the majority of people. Without a referendum, a constitution, no matter how well it's written, lacks legitimacy," he said.
"I personally think that if the NCPO (National Council for Peace and Order) or the government think it's not good, it can use its power under Section 44 (of the interim charter) to scrap it, without having to waste 3 billion baht on a referendum.
"Another way is for the NRC to vote down the draft but there's little chance this will happen since the NRC will also be dissolved (if it votes against the draft).
"Alternatively, the interim charter could be amended to stipulate a referendum after the NRC endorses it. It all depends on the NCPO."
Prime Minister Prayut Chan-o-cha has said that he personally is not ready to even discuss a referendum until the NRC makes its decision on the draft charter in August.
However, an NCPO spokesman commented recently that a referendum could be socially divisive as political parties and other interest groups could use it to stir up old grievances.
As well, a referendum would result in an election, expected around February next year, to be pushed back by three months or more.
The PDC, an independent unit under King Prajadhipok's Institute, was set up in 2008 under the 2007 constitution. Its duty is to promote political development and improvement of morality and ethics among politicians and government officials.
Its members are representatives from social organisations chosen by the assembly of tambon organisations from all political parties, among others.