BANGKOK (AFP) - Thailand's junta-picked parliament Thursday will vote next week on the impeachment of 248 ex-lawmakers, the majority from the ousted ruling party of Yingluck Shinawatra, in a move that could decimate her family's political base.

The former MPs face a five-year ban from politics if the National Legislative Assembly (NLA) votes in favour of impeachment on 14 August, threatening to remove more key allies of the Shinawatra clan whose parties have swept every election since 2001.

The ex-lawmakers are accused of supporting an amendment to the country's charter aimed at making the upper house a fully elected chamber that was later deemed unconstitutional.

If impeached they would follow in the footsteps of Yingluck, who fell foul of the NLA in January over a separate issue.

"The assembly has finished hearing the case against the former MPs" and will vote next Friday, NLA speaker Jetn Siritharanont told AFP.

A successful impeachment requires 132 out of 220 assembly members to vote in favour, he added.

Thailand's generals say their May 2014 coup was necessary to restore order after months of often violent anti-Yingluck protests paralysed Bangkok.

But critics say it was the latest attempt to claw back power from the Shinawatras for the kingdom's elites and their supporters in the military and judiciary.

Anusorn Iamsa-ard, deputy spokesman for Yingluck's Puea Thai party, said the party was "not concerned" over the impeachment.

"The standards have been set by earlier cases where no one was impeached," he told AFP, referring to a NLA vote in March when 38 senators were not impeached over the same issue.

Yingluck, Thailand's first female premier, was impeached and banned from politics for five years in January over her administration's controversial rice subsidy programme.

She also faces up to 10 years in jail in an ongoing criminal trial on accusations of criminal negligence over the populist scheme, which paid farmers in the rural Shinawatra heartland twice the market rate for their crop.