BANGKOK (AFP) - Thailand's anti-graft body on Thursday launched impeachment proceedings against 250 former lawmakers, the majority from the toppled ruling party of Yingluck Shinawatra, in a move that could be a bodyblow to her family's political network.

If the junta-picked National Legislative Assembly (NLA) votes for impeachment, the former MPs will be banned from politics for five years, eviscerating the political base of the Shinawatra clan, whose parties have swept every Thai election since 2001.

The National Anti-Corruption Commission (NACC) says a 2013 attempt by Yingluck's Peau Thai party to tweak the kingdom's charter to make the Upper House a fully elected chamber was unconstitutional.

"The NACC decided to file case to the NLA to impeach 250 MPs who signed up to support charter amendment because they intended to breach the 2007 Constitution," the anti-graft body said in a statement.

The decision came just hours after the NLA voted not to impeach 38 former senators over the same issue, initially appearing to lessen the likelihood of the NACC pressing ahead with impeachment proceedings for the lawmakers.

As yet there is no date set for the assembly to vote on the impeachment.

Thailand's junta says it was forced to take power in May last year to restore order after months of street protests against Yingluck's government.

Critics say those protests were choreographed to prod an army intervention on behalf of the Thai elite, who accuse the Shinawatra clan of poisoning Thai politics with money and nepotism.

Peau Thai member Amnuay Klangpa told AFP that 160 party members were in the firing line after the NACC decision.

"I am confident they cannot impeach us because MPs had the right to amend the constitution and all processes were carried out within the law," he added.

Retroactive impeachment has already been brought down on Yingluck - the younger sister of billionaire former premier Thaksin Shinwatra - over her role in a bungled rice subsidy scheme.

She also faces criminal prosecution carrying up to 10 years in prison over the scheme, which funnelled cash to her family's rural support base.

Thailand's political landscape has been battered by nearly a decade of violent street protests, coups and legal manoeuvres leaving the country deeply divided.

Thaksin sits at the heart of the kingdom's political rupture after sweeping the 2001 polls with his pledge to represent Thailand's poor and drive the nation's economy.

Thaksin was toppled by a 2006 coup and now lives in self-exile to avoid jail on a corruption charge.

His sister Yingluck was ousted by a court decision eight years later, weeks before the latest coup knocked out the remnants of her government.