A Seoul court on Friday upheld a travel ban on Morse Tan, a former Korean American professor charged with defaming President Lee Jae Myung, preventing him from leaving Korea until Aug. 15.

The ruling was made after Tan filed a suit to overturn the exit ban the justice ministry had imposed on him for the third time amid an investigation into a defamation case involving his allegedly false claim that Lee was sent to a juvenile detention center as a teenager for taking part in a murder.

The Seoul Administrative Court ruled against Tan on Friday, saying the disadvantage to the plaintiff from maintaining the ban did not greatly outweigh the public interest.

The court also determined the former professor might not return to Korea to face additional questioning and potential indictment if he were to leave the country, given his uncooperative attitude toward the judicial proceedings.

After arriving in Korea on May 28 to observe the June 3 local elections, Tan did not comply with a police summons to appear for questioning, triggering the justice ministry's first travel ban on him through June 30.

Following the police's transfer of the case to the prosecution, a second exit ban was implemented through July 31. The restriction was renewed after the prosecution indicted Tan on July 16.

"The judiciary has abandoned its duties and proved the collapse of the separation of powers and the constitutional order, as well as the death of the rule of law," Tan's lawyer said at a press conference, vowing to appeal as soon as possible.

The first formal hearing of Tan's defamation trial is scheduled for Sept. 11.