The National Reform Council (NRC) expects to vote on the draft charter in early September, NRC chairman Thienchay Kiranandana says.
Mr Thienchay said the vote could take place during Sept 5-7 assuming the NRC accepts the draft in the third week of August.
The interim charter requires a vote to be held within 15 days of the draft acceptance. He said the NRC will not hold a debate on the draft charter before it votes to accept or reject it.
The timeline is based on an amendment to the interim charter which allows the Constitution Drafting Committee (CDC) to extend the drafting period by no more than 30 days.
Asked about a referendum, the NRC chairman said the question will be discussed on the day before the vote is cast because the NRC will be dissolved right after the vote.
One of the seven amendments to the 2014 charter allows the NRC and the National Legislative Assembly (NLA) to ask one question each when the draft constitution is put up for a referendum.
He said the referendum question should be related to the draft constitution and a question on whether the term of an organisation can be extended is deemed relevant.
"The question is wide open. But some are improper to ask, for example, who the next prime minister should be. So we need to come up with a question that suits the current situation and the next six months to 12 months.
"A proposal to extend the term of the government could be put as the question and it is relevant to the charter," he said.
Mr Thienchay said the NRC will propose three reform packages before its term expires: the country's vision; reform agendas and reform areas deemed top priorities; and the restructuring of bureaucratic and national administration.
Meanwhile, the Election Commission decided yesterday that around 50 million voting cards will be required for each question of the referendum, which is expected to be held early next year.