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 August 7 vote.
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.
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"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.
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.