Seoul prosecutors on Tuesday appealed the recent court ruling that acquitted a former Yonsei University professor of defamation in relation to his claim that the Korean victims of Japan's wartime sexual slavery were voluntary prostitutes.

Last week, Seoul Western District Court found Lew Seok-choon not guilty of defaming the victims, euphemistically called comfort women, in a lecture five years ago.

"Considering that (Lew's comments) are in contrary to the historically proved fact, that there is a limit to the freedom of learning mentioned in the Constitutional Court, and the fact that he wasn't merely presenting his personal belief but presenting (the comments) as facts, the court's decision is wrong," Seoul Western District Prosecutors' Office said.

While Lew had been cleared of defaming the comfort women, the court fined him 2 million won ($1,500) for a separate charge of defamation on Korean Council for the Women Drafted for Military Sexual Slavery by Japan. He had said that the civic group trained the victims to testify as if they had been forcibly drafted by the Japanese military. The prosecution said this punishment was not strong enough.

Lew, a 69-year-old right-wing scholar who formerly taught sociology at one of the top schools in South Korea, said during a Sept.19, 2019 lecture that comfort women were "kind of prostitutes." But the court said in its ruling that his comments were his personal beliefs, and within the constitutional rights of all citizens to "enjoy freedom of learning and the arts," stipulated in Article 22 of the Constitutional Law.