The 75-year-old former media regulator was grilled by prosecutors for more than 14 hours over allegations he received kickbacks from a real estate developer in exchange for wielding his influence in a major development project.

Choi, often called Lee's "political mentor," has admitted to taking money and spending it on conducting public opinion surveys during Lee's 2007 presidential campaign. "I would like to offer an apology not only to Cheong Wa Dae but to the public,"he told reporters, referring to the presidential office. He did not disclose any details.

Prosecutors are reportedly seeking an arrest warrant against Choi.

Earlier this year, Choi stepped down as the head of the state- run Korea Communications Commission to take responsibility for a separate bribery scandal involving his close aide.

The graft scandal follows on the heels of a series of corruption scandals implicating Lee's aides and relatives, an embarrassment for the conservative president who has prided himself on the "morally perfect" administration.

Lee's Saenuri Party, which recently won an upset victory in general elections after having successfully distanced itself from the president, is calling for a thorough investigation into the allegations.

The ruling party is currently headed by Park Geun-hye, a presidential hopeful and political nemesis of Lee, whose five-year term ends early next year.

The main opposition Democratic United Party, meanwhile, said Choi received illegal political funds and the scandal should not be branded as just another corruption case.