NACC secretary-general Sansern Poljiak said the agency has called into question Bancha Porameesanaporn's professional conduct following remarks made on Tuesday involving the NACC's handling of the rice-pledging case against Ms Yingluck.
Speaking to the media, Mr Bancha criticised the NACC decision not to allow seven witnesses to testify in her defence as unfair and improper. He threatened to sue the NACC members for abuse of authority, saying his team was gathering evidence against them.
Mr Sansern said the agency's probe against Ms Yingluck, who is accused of dereliction of duty for her failure to tackle allegations of graft in the rice-pledging scheme, is not yet finalised. Her lawyer's remarks are uncalled for and deemed acts of intimidation, he said.
He said the NACC will ask its office to lodge a petition with the LCT against Mr Bancha for breach of etiquette.
The anti-graft agency argued it could not allow the seven witnesses to testify in Ms Yingluck's defence because they are unrelated to the issues with which Ms Yingluck has been charged.
The witnesses include caretaker Labour Minister Chalerm Yubamrung, deputy national police chief Worapong Chiewpreecha, caretaker PM's Office Minister Varathep Rattanakorn, accountant Phichai Choonhavajira, and rice-scheme adviser Orlarn Chaiprawat.
Findings of the rice-pledging scheme case against Ms Yingluck are expected to be discussed by a panel on Tuesday. If the panel deems its evidence sufficient, the case will be concluded and submitted to the anti-graft commissioners.
The NACC is expected to rule on the case on Thursday or May 13. Ms Yingluck stands to lose her premiership if the commission finds her guilty.