Two members of the National Reform Council (NRC) have indicated they will vote to reject the draft charter.
The Constitution Drafting Committee (CDC) handed over the final draft to the NRC on Saturday, which will vote on it on Sept 6.
If the council approves the draft, a referendum to gauge the public's reaction is expected to be held in January.
If it rejects the draft, the junta will appoint a new committee to draft another charter within 180 days.
Regardless of the outcome, the NRC will be dissolved after the vote.
The draft has 285 sections, many of which have been criticised. These include a provision that allows for the appointment of a non-elected prime minister, and a proposal to establish a so-called "crisis panel".
The panel will have special powers to intervene in politics by seizing both executive and legislative powers from an elected government and parliament, if such action is deemed necessary.
Sombat Thamrongthanyawong, who chairs the NRC's political reform panel, said the draft charter encourages the setting up of a coalition government comprising small parties.
He said this would lead to a weak and unstable government prone to collapse.
The draft charter favours smaller and medium-sized parties and increases their bargaining powers, and a prime minister would have no power to control cabinet ministers from different parties, Mr Sombat said.
Decision-making in a coalition government often needs the support of other coalition parties, compared to a government that has won a majority in the House and has more unity, he added.
Mr Sombat said the executive branch has a central role in running the country and it should not be weakened by the draft charter.
Reforms may not succeed if the new charter is allowed to "clip the wings" of future governments, Mr Sombat said.
"The draft charter is likely to create problems. I will not let it pass if the draft ruins the country," Mr Sombat said.
NRC member Wanchai Sornsiri said Sunday he did not think the document could address the country's problems and he would reject it.
He said the draft charter was undemocratic and unlikely to reform the country as promised, and also described the so-called crisis panel, which critics argue is undemocratic, as a "malignant tumour".
Instead of preventing a conflict, the panel would create a crisis and hinder the reform process, Mr Wanchai said.
He also said the draft charter did not put in place any mechanisms to ensure future elections were free of fraud.
Mr Wanchai added the NRC's political reform committee and justice reform committee will hold a joint meeting today to consider the draft and will declare whether they will accept or reject it.
He also said NRC members and the CDC will attend a seminar tomorrow in which each NRC member will be able to make a decision on whether to vote for or against the draft, after hearing the CDC's comments on the final draft.
