The Democrat party has declared its intention to oppose the draft charter as well as the additional question at the planned charter referendum this August.
They warned a "yes" vote to the second question, which would pave the way for an appointed Senate to intervene in the selection of a prime minister, could escalate political conflicts.
The party's stance follows a vote by the National Legislative Assembly (NLA) last week to add a second referendum question, asking the public whether parliament should jointly approve a prime minister for a five-year transitional period after a general election.
Citizens will be expected to vote on it at the Aug 7 referendum, along with a question on whether the charter draft prepared by the Constitution Drafting Committee (CDC) led by Meechai Ruchupan should be adopted.
If the extra question gets a majority "yes" vote, it will force the Meechai panel to revise the constitution draft which currently does not allow the upper House to choose the prime minister.
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Addressing a media briefing, Democrat leader Abhisit Vejjajiva said the party rejects the second question on the basis the senators are handpicked by the National Council for Peace and Order (NCPO).
"Their selection of a prime minister will be against the people's will and will contradict the principles of the CDC," he said.
The public will lose if this is the case, said the former prime minister.
The Democrat leader said the major political parties will have to join hands to ensure appointed senators do not get to decide who the prime minister will be.
He said that would have a bigger effect as it will give them a majority in parliament.
Mr Abhisit said the party has not made a decision on details of the charter draft but several members agree it contains many flaws and is not fit to be adopted as the country's supreme law.
One of the major flaws is it reduces the people's powers, he said.
The Meechai draft is inferior to the 2007 charter in every aspect, said Mr Abhisit. He said certain clauses concerning the right to education, health and legal services and consumer and environmental protection laws could cause conflicts in the development process, particularly in the implementation of megaprojects.
He said while the charter draft is designed to rein in corruption, it drops the need for impeachment and opts for a court trial which would make it difficult to tackle corruption.
Moreover, it would be difficult to amend the new charter, especially the clauses on the appointed Senate, as the move requires support from at least one-third of senators who are all appointed by the NCPO, he noted.
The five-year provisional clauses of the charter would mean the appointed Senate would serve longer than the House of Representatives and involve two elected governments, he said.
"The Democrat Party believes the charter has more disadvantages than advantages. We disagree with this constitution. The country doesn't need to choose between dictatorship and corruption," Mr Abhisit went on to add.
Mr Abhisit also called on the NCPO to outline the next steps in case the charter draft is rejected in the referendum, saying it should do so as soon as possible.
"It's still hard to say if we will vote for it or against it. Some people reject it before it is complete. Some people support it no matter what, because they don't like the people who reject it."
"We can't afford to be in this situation. The NCPO should assure everyone that no matter what the outcome of the referendum, we won't be headed towards another violent conflict," he said.
Prime Minister and NCPO chief Prayut Chan-o-cha on Sunday shot back at critics of the second referendum question, especially politicians, saying they were afraid the NCPO would succeed in implementing reforms and people would not vote for them.
"They pay no attention to, nor make any effort to understand, the NCPO's work even though we are doing good things to bring about changes. That's fine. But they are critical of our efforts in implementing reforms. That's because they are afraid once we succeed, people won't elect them to office," he said.
He also said the politicians hardly solved any problems but were in fact a part of them.
Government spokesman Sansern Kaewkamnerd has shrugged off criticism of the extra question, saying it is intended for the whole country, not just certain groups of people.
He expressed confidence the Election Commission would be able to hold an impartial referendum.
