Wilas Chanpithak, a former MP of the Democrat Party, says he will not vote for the new draft constitution prepared by the Meechai Ruchupan committee, saying it is less democratic than the 2007 charter, according to media reports.
Mr Wilas said he would exercise his voting right in the Aug 7 charter referendum, but would not vote in favour of it.
He also would not vote for an additional question on a proposal for appointed senators to join MPs in voting to elect the prime minister.
"In principle, a new constitution should be more democratic than the old ones. This draft charter is less democratic than the 2007 charter," he added.
Mr Wilas said this was based on his own opinion and he had not talked it over with other members of the Democrat Party, who he said might hold different opinions from his.
On British Prime Minister David Cameron's vow to resign to show responsibility after the Remain vote was beaten in the Brexit referendum, Mr Wilas said it would be good if Thai politicians could hold the same standard. "Maybe that would help uplift our politics," he added.
Asked whether he meant to say that Prime Minister Prayut Chan-o-cha should resign if the draft charter failed to pass the referendum, Mr Wilas said he could not make a decision on behalf of other people, adding that he only raised Mr Cameron's case as an example.