SEOUL (AFP) - South Korean prosecutors have decided not to appeal against the acquittal of a Japanese journalist who was charged with defaming President Park Geun Hye, citing diplomatic ramifications, a prosecution official said Wednesday (Dec 23).

It was an extremely rare decision as prosecution authorities have previously made it a rule to bring lost cases to a higher court.

The Seoul Southern District Court said Friday (Dec 18) that Tatsuya Kato, former Seoul bureau chief for Japan's Sankei Shimbun daily, defamed the president's honour as a private citizen by writing and publishing an article questioning where Park was on the day the Sewol passenger ferry sank, killing 300 people in April last year.

The article picked up unproven rumours circulating in the South Korean media that the unmarried Park had disappeared for a tryst with her former aide when the boat sank off South Korea's southern coast.

However, the court declared Kato not guilty, noting that he wrote the story to serve the public interest and the freedom of the press should be guaranteed in a democratic society.

"We've decided not to bring the case to an appeals court as it has become clear in the course of the lower court trial that the report was inaccurate and the foreign ministry has also asked for leniency for the sake of the development of South Korea-Japan ties", the official was quoted as saying by Yonhap news agency.

Seoul prosecutors had demanded a guilty verdict and an 18-month jail term for Kato, arguing he had intended to defame Park.

Kato and the newspaper welcomed the verdict, saying prosecutors should respect the court's decision and give up any attempt to appeal.