Prosecutors have decided to reopen an investigation into an illegal surveillance scandal involving the Prime Minister's Office two years ago, following a recent allegation by one of the key suspects that the presidential office attempted a cover-up, a source at the prosecution said Friday.

Seven officials at the office were indicted in 2010 in connection with the unauthorized surveillance operation of a businessman who posted a video clip critical of the government of President Lee Myung-bak on his blog in 2008. They appealed and are awaiting rulings by the Supreme Court.

One of the seven officials on trial, Jang Jin-su, recently revealed that he destroyed hard drives and computer data under the direction of an official at the presidential office Cheong Wa Dae before prosecutors launched the probe.

Jang claimed that the direction was given by Choi Jong-seok, a former labor affairs official at the presidential office by telephone, disclosing a recording of phone conversations between Jang and Choi.

With the new allegation, prosecutors have internally decided to reopen the probe, but are weighing when to launch a reinvestigation because it could affect the April 11 parliamentary election, the source said on the condition of anonymity.

In the disclosed phone conversation, Choi also encouraged Jang to destroy evidence with a "hammer and dump it into the Han River,"

saying the presidential office had a deal with the prosecution, according to local media reports.

Opposition parties have demanded prosecutors launch a reinvestigation into the illegal surveillance scandal. (Yonhap News)