Constitution Drafting Committee (CDC) chairman Bowornsak Uwanno insists all proposals from various reform committees, including the proposed direct election of a prime minister, are not yet final and the process has a long way to go.
He was responding to criticism suggesting the plan will give a directly elected premier too much power and weaken the checks-and-balances system.
He previously said the process would continue until mid-April next year, and the new draft charter should take shape by the end of July.
Meanwhile, the chairman of the political reform committee under the National Reform Council (NRC) Sombat Thamrongthanyawong said his committee's proposal of a direct election for prime minister and the cabinet would be the world's first "political innovation".
The committee yesterday offered new details of its proposal, covering everything from the legislature to the cabinet.
Mr Sombat said each political party will nominate candidates to run in the election of the prime minister and the cabinet.
This proposal means, when a prime minister is elected to the House, he or she will have some support already from the House of Representatives, which must approve the voters' preference. He said if the party whose candidate wins in the premiership does not win the majority House seats to form a government, it can persuade other parties to form a coalition.
A candidate must win support from more than half of the voters to become prime minister, Mr Sombat said. If no candidates win the required support, a runoff will be called.
"This is a political innovation which Thailand may be the first country in the world to use," he said.
Democrat Party deputy spokesman Ramet Rattanachaweng yesterday opposed the direct election of a prime minister, saying a powerful executive branch has led to political problems in the past, as it refused to come under scrutiny from independent organisations.
He urged the committee to establish a more efficient checks-and-balances system to keep the executive in check.
Noppadon Pattama, a key figure of the Pheu Thai Party, also questioned the proposal on his Facebook page, saying there is no guarantee there would not be massive vote-buying in the lead-up to the premier's election.
Regarding the proposal for a directly elected cabinet, Mr Noppadon questioned whether the prime minister would have the power to make personnel changes to the cabinet.
Mr Sombat unveiled a litany of other proposals yesterday. He said that when a royal decree is issued to set the date for a general election, ministerial permanent secretaries would assume the duties of cabinet ministers, and act as a caretaker government.
A permanent secretary will also be chosen from among them to assume the duties of a prime minister.
The duties would include submitting the names of a new prime minister and new cabinet ministers for royal endorsement, Mr Sombat said.
Regarding the legislative branch, Mr Sombat said the committee agreed to retain the bicameral parliamentary system, consisting of the House of Representatives and the Senate.
The House would comprise 350 MPs, all of whom will be elected under the constituency system, while the party list system will be rescinded to protect political parties from being dominated by businessmen and political party financiers, Mr Sombat said.
Each constituency will have no more than three MPs and a voter can vote for only one MP.
The Senate will have 154 members. Half of them will be elected, one from each province. The rest will be elected from among profession groups, Mr Sombat said.