An arrest warrant has been sought for a sitting prosecutor accused of conspiring with the main opposition party to investigate key ruling camp officials last year when Prosecutor-General Yoon Seok-youl, now a leading opposition presidential contender, was in office.

The Corruption Investigation Office for High-ranking Officials (CIO) said it sought the warrant over the weekend for Son Jun-sung, who served at the Supreme Public Prosecutors Office last year, as he has not cooperated with the investigation.

The scandal centers on allegations that Son asked the main opposition People Power Party (PPP) to file a complaint against three pro-government figures just before the general elections in April last year so that the prosecution could launch investigations.

Son was accused of asking Kim Woong, a former prosecutor and now a PPP lawmaker, to lodge a defamation accusation against the three, including Rhyu Si-min, a liberal commentator and former chairman of the Roh Moo-hyun Foundation.

The CIO said it has repeatedly asked Son to appear for questioning but he has been uncooperative, offering "unconvincing reasons" for rejecting the summon requests.

Investigators in charge of the case reportedly confirmed Son's involvement after a team analyzed a massive trove of digital data and documents obtained from a whistleblower and the SPO.

The CIO has formally booked Yoon on multiple charges, including abuse of power, in its probe into the scandal.

Yoon has denied involvement in the case. (Yonhap)