"The upcoming hearing into alleged tax evasion appears to be unfair because two key investigators seem to have pre-judged the outcome," lawyer Theera Thammasit said.

The petition was filed with the Assets Examination Committee (AEC), asking it to review its Sak Korsaengruang panel.

Panel chairman Sak and panellist Klanarong Chantik should be removed on grounds they were bent on faulting Thaksin instead of ensuring justice, Theera said in the petition.

The Sak panel is poised to open its hearing into the Shin shares transaction involving Thaksin's wife Khunying Pojaman, her brother Bhanapot Damapong and her nominee Duangta Wongpakdee.

The three will be allowed to present their defence before the panel moves on to the indictment phase of the graft proceedings.

The hearing is based on the fact-finding report from the Viroj Laohaphan panel last month uncovering evidence that the Bt738-million transaction was not entitled for tax exemption as a family gift as previously claimed by the Shinawatra-Damapong families.

The Viroj panel instructed the Revenue Department to collect capital gains tax and related tax liabilities from Bhanapot. The case is being contested in the departmental review and the defence has vowed to fight for a Supreme Court decision.

Thaksin's chief defence lawyer Noppadon Patama said he had assigned Theera to handle tax-related cases.

"The AEC appears to have its sword ready to strike and the defence has no choice but to rely on every available legal shield," Noppadon said, explaining why the defence wanted changes in the Sak panel.

He also stated that Thaksin and his family members would fight every case to the fullest extent of the law.

Even before the hearing starts, Sak and Klanarong have said in news reports that Bhanapot had intentionally evaded his tax obligations, he said.

Reacting to the petition, Klanarong said the defence had the right to question his integrity - but the AEC would have the final say on the composition of the Sak panel.

The nine-member AEC is expected to debate the petition next week.

Sak said he expected the AEC to continue endorsing his chairmanship as the petition had no tangible evidence to doubt his impartiality.

Should the AEC decide to revise the panel's composition, the defence will have seven days to accept or oppose the new set-up.