Judges have dismissed a plea for justice by Yingluck Shinawatra, who claims that defence witnesses in her trial are being intimidated by the National Anti-Corruption Commission.
The former prime minister outlined her complaint on Friday in a letter to the Supreme Court's Criminal Division for Political Office-Holders, where she is being tried on charges of negligence in connection with a rice-pledging scheme that cost taxpayers hundreds of billions of baht.
Ms Yingluck claimed she had not been treated fairly by the NACC and the prosecutors.
She said that Supa Piyajitti, a former deputy permanent secretary for Finance, who was a prosecution witness, is now an NACC commissioner and a member of the anti-graft agency's investigation panel.
She said it had become a pattern that after her witnesses had testified, the NACC would set up a panel to investigate each one of them.
They include Yanyong Puangraj, a former commerce minister; Kittiratt Na Ranong, a former deputy prime minister; and Adul Yuwawitthayapanich, a legal adviser to the chairman of the Public Warehouse Organisation.
Another defence witness who has yet to testify was also intimidated by Ms Supa, the former premier said.
"I therefore plead for justice. If this keeps happening, nobody will dare testify for me as they fear charges," said Ms Yingluck.
The court dismissed the petition, saying it did not have the power to order the NACC to investigate or not to investigate anyone.
The court on Friday heard testimony from Yukol Limlaemthong, a former agriculture minister.
Witness testimony in the case is scheduled to continue until next month with a ruling expected sometime later this year.
Ms Yingluck could face 10 years in jail and a ban from politics if found guilty of the charges she faces.