Ousted Thai premier Yingluck Shinawatra on Friday urged the ruling military junta to state whether it will hold elections next year as planned if Thais vote down a new charter in a looming referendum.

The generals who kicked Yingluck's government out in 2014 are preparing to put their controversial document to the people in August, marking Thailand's first return to the polls in over two years.

But the junta has been mute on what will happen if the charter fails to pass, sparking concern that a promised return to democracy in 2017 could be further delayed.  

"I urge the NCPO to clearly announce what will happen after the referendum," Yingluck tweeted in rare political comments on Friday, referring to the junta's official name, the National Council for Peace and Order.

She added, "will the NCPO hold a general election as it promised to the international community according to the announced roadmap?"

The military has repeatedly pushed back the timetable for a return to democracy.

Yingluck and the powerful political machine run by her billionaire brother, fellow ex-premier Thaksin Shinawatra, have kept unusually quiet for much of the past two years, fenced in by the junta's ban on political organising of any kind.

But the party now appears to be lurching back to life as it launches a campaign to reject the charter.

Critics say the document will enshrine military rule under the guise of a constitutional democracy, slamming it as a naked effort to limit the influence of elected politicians - a game dominated by the Shinawatras.

The military-appointed drafters have hailed their work as a solution the kingdom's "lost decade" of political deadlock, which has seen the Shinawatra's rural and largely poor supporters struggle to wrest power from a military-backed elite.

Under the proposed charter, a junta-appointed senate with seats reserved for military commanders would check the powers of lawmakers for a five-year transitional period following fresh elections.

The junta has also warned it will not tolerate criticism of its work in the run-up to the vote, already detaining two opposition politicians for challenging the document this week.

Yingluck is also facing a decade in jail over a negligence charge linked to costly farming subsidies doled out while she was in office.