BANGKOK (AFP) - A former prime minister whose party supported Thailand's last coup lambasted the junta's new constitution on Wednesday (July 27) before a referendum on it, a rare blow to the army from within its own political camp.

Abhisit Vejjajiva, who was appointed prime minister from 2008-2011, leads the Democrats, Thailand's second biggest party.

They have failed for two decades to win an election but carry major clout within the Bangkok establishment that rallied behind the May 2014 overthrow of Yingluck Shinawatra's elected government.

The army has held the country in lockdown since its coup, banning protests, muzzling dissent and even outlawing campaigning against the charter before the Aug 7 vote.

The junta says the new document is crucial to ending a decade of political turmoil that has frequently spilt into violence.

But critics say it will straitjacket democracy with clauses calling for a fully-appointed senate and unelected premier - both of which could help the military keep its allies in power.

"I do not approve of the draft constitution," Abhisit told AFP in a rare attack on the junta from within the powerful establishment.

"It goes against the basic principle of what we believe in... democracy," he said, adding that the document "will trigger new conflict".

He urged the junta to rewrite the charter.

Abhisit, a smooth speaker educated at top British fee-paying school Eton, is a serial election loser.

He lost elections in 2007 but was appointed prime minister by parliament a year later as the Shinawatra-allied government was hit by a legal ruling.

He lost another vote in 2011 which swept Yingluck into power, and also led his party in boycotting elections in 2006 and 2014.

In 2013 Abhisit's deputy Suthep Thaugsuban led the protests to unseat Yingluck.

Abhisit was a vocal supporter of the protests but became uncharacteristically taciturn as the coup unfolded.

His intervention against the referendum is likely to sting a military which is already hypersensitive to criticism.

Last week two eight-year-old girls were charged under a junta law for tearing down voter lists in a game, the latest increasingly bizarre act in a crackdown before the vote.

Yingluck's toppled Puea Thai party has also urged supporters to vote down the charter next month - in what will be the first political act since the coup.

Thailand remains deeply divided.

The royalist Bangkok elite despises the Shinawatra family, whose parties have won every election since 2001, accusing them of poisoning the country with graft and nepotism.

But the Shinawatras, headed by Yingluck's billionaire elder brother and former prime minister Thaksin, retain major support among the rural poor and urban middle class.