Any achievements by Thailand's ruling junta while in power will be lost without significant changes to the country's draft constitution, Democrat Party leader and former prime minister Abhisit Vejjajiva said on Friday.
The military toppled an elected government last May and is engineering wide ranging reforms to heal the country's deep political divisions.
But both the Democrat Party and its rivals have called the junta's draft charter undemocratic, saying it could herald more turmoil under a weak coalition government.
"If they don't come up with a constitution that is accepted, and cannot come up with one that can be sustained, they will have lost everything," Mr Abhisit, who was prime minister from 2008 to 2011, told Reuters in an interview.
"Whatever they have achieved will just come to nothing."
Drafters have worked on the false assumption that weakening political parties would prevent abuse of power and stem corruption, Mr Abhisit said.
But a weakened party system would play into the hands of politicians such as exiled billionaire and former Prime Minister Thaksin Shinawatra, he added.
"He's got the resources to draw politicians away from political parties into groupings that could actually form a new coalition," he added.
Thailand has been divided for more than a decade between supporters of Thaksin and the Bangkok-based royalist-military establishment, which reviled Thaksin's popularity.
Mr Abhisit said his party had "never had a problem" with Thaksin's Pheu Thai Party but that it should take the opportunity to move away from the "Thaksin agenda".
He added, "I've always maintained that the conflict has been between Thaksin and the Thai state. That should give a clue to the people in power, as well that the secret of reconciliation is to get beyond the Thaksin issue."
A year after then army chief Prayut Chan-o-cha launched the coup to restore order following months of protests, Mr Abhisit said the junta still had a long way to go to tackle Thailand's political problems.
"What I would warn the current regime is that I don't think they have been able to get to grips with what was wrong with the country before May 22. They still have time to rectify that."
Wide public debate was needed on the constitution and it would be better to delay elections set for early next year if necessary, to get the charter right, Mr Abhisit said.
"There is no way they can correct what I see as mistakes until they open a forum and open their minds," he added.