Lawyers for jailed Khmer Rouge leader Ieng Sary filed a petition to the tribunal courts Monday protesting the legality of a trial due to an earlier royal amnesty.
Lawyer Ang Udom said Wednesday a trial of his client would be illegitimate.
"He cannot be tried for the second time," Ang Udom said. "This is contrary to local and international law, because he has been tried and pardoned by the king."
Ieng Sary was found guilty in absentia of genocide in a trial following the ousting of the Khmer Rouge in 1979. The trial is widely viewed as a show trial.
Ieng Sary defected to the government in 1996, following a royal pardon and amnesty by then king Norodom Sihanouk for the guilty verdict of the Vietnamese trial.
Lawyers argued the amnesty should take precedence over the hybrid courts. However, experts say he can still be charged with war crimes, even if the courts allow the amnesty argument.
Tribunal co-prosecutor Robert Petit confirmed Wednesday receipt of the petition, which said the court had no authority in the Ieng Sary case, due to the royal pardon and amnesty agreement with the government.
"Indeed, we will look at that closely and reply to the lawyers in writing," he said.