Prosecutor General Kim Oh-soo on Thursday ordered an investigation into allegations that a prosecutor asked the main opposition People Power Party to file a complaint against three ruling bloc figures when Kim's predecessor Yoon Seok-youl, now a leading opposition presidential contender, was in office.

In a text message to reporters, the Supreme Prosecutors Office said Kim instructed its inspection department to look into the allegations.

Earlier Thursday, online news outlet Newsverse reported that prosecutor Son Jun-sung asked Kim Woong, a former prosecutor and now a People Power Party lawmaker, to lodge a defamation accusation against the three men just before the general elections in April last year.

At that time, Yoon, currently seeking the People Power Party's ticket to run in the March presidential election, was the prosecutor general.

The complaint form delivered by Son to Kim targeted Rhyu Si-min, a prominent figure of the liberal bloc, Choe Kang-wook, a current lawmaker of the minor liberal Open Democratic Party, and Hwang Hee-seok of the same party, as well as seven journalists, according to the news report.

The complaint accused them of involvement in news reports alleging that Yoon's wife was implicated in stock manipulation and that a TV reporter and a key Yoon aide colluded to blackmail an inmate to dig up corruption involving Rhyu, the report said. 

Kim, then a parliamentary candidate, passed the request to the party, according to the report, but the party did not file the complaint.

Both Son and Kim denied the report.