Liberal presidential front-runner Moon Jae-in called for an investigation on Sunday into allegations that the National Intelligence Service has spied on the Constitutional Court in the runup to its ruling on President Park Geun-hye's impeachment.

   Citing a former high-rank NIS official, SBS reported a day earlier that the spy agency has been gathering information on the Constitutional Court as justices move to deliver a ruling on the impeachment.

   The Constitutional Court is widely expected to announce its decision whether to officially approve Park impeachment as early as Friday. The National Assembly impeached Park in December on charges of influence peddling and corruption.

   "According to the media, an NIS official said the agency has been spying on the Constitutional Court since January," said Park Kwang-on, election spokesman for Moon of the main opposition Democratic Party.

   "If the report is true, it constitutes a serious breach of official discipline," the spokesman said, calling for an immediate prosecution investigation into the case.

   "Who has ordered the spying, in whose hands the collected information has ended up and whether any Constitutional Court official has been informed before or after the spying should be clarified," he stressed.

   When asked by Yonhap News Agency, however, an NIS official dismissed the spying allegation as "groundless."  (Yonhap)