It claimed that Song's memoir contained false accusations that Moon exerted influence to prevent South Korea from voting for a UN resolution on North Korea's human rights. Moon was then chief of staff for President Roh Moo-hyun.

Song, who took the ministry's helm from 2006-2008, wrote in his memoir that the government asked the North of its opinion before it decided to abstain from the UN vote.

Prosecutors verified Moon aides' claim that Cheong Wa Dae had made up its mind before it talked with the North.

But it also concluded that it was difficult to charge Song with defamation, since there are sufficient grounds to believe that he had been misled about the situation.

Prosecutors also said Song could not be charged with revealing government secrets since the documents published in the memoir are neither original nor recent enough to constitute a violation of such law.

Moon became the president in May, following the ousting of his predecessor Park Geun-hye over an influence-peddling and corruption scandal.

The book, published only weeks before the May presidential vote, sparked a huge controversy and fierce attacks on Moon from his rivals, who accused him and the Roh government of secretly communicating with Pyongyang about the vote. (Yonhap)