National Reform Council (NRC) appointed by military on Sunday rejected a controversial new constitution drafted following last year’s coup.
Critics describe the constitution as divisive, anti-democratic and aimed at prolonging military rule. The constitution was believed to enable a panel to collapse government during a national crisis.
After Sunday's vote the toppled Peau Thai party criticised the constitution as "dictatorial" saying it would destroy democracy.
In recent weeks, disagreements surfaced among the council members as the constitution was expected to pass through the panel. The biggest disagreement was about the creation of a committee that would have been dominated by the military. It would help to exercise power over the executive and legislative branches in a crisis situation.
The constitution received 105 votes, as it was supposed to be 125 to pass. Hundred and thirty-five were against the document.
"Those in favour were less than half of NRC members, it means this NRC meeting has voted against" the charter, NRC Chairman Tienchai Kiranan said in a televised count.
The junta has one month to appoint another committee that will have 180 days to write a new constitution and after three months a nationwide referendum will be held. These processes will push the timetable for elections to 2017.
"They will then draft a new charter within 180 days and then it will go to a national referendum," NRC member Paiboon Nititawan told AFP.
"It will delay elections about six or seven months," he added.
Analysts said the rejection of the constitution would be politically helpful for the junta as it postponed the date for a return to civilian rule.
The army deposed the elected government and seized power in May, 2014, promising to repress the uprising, protests and acrimony by political rivals after months of unrest.