South Korea's main opposition has admitted it is implicated in a scandal that has already threatened the country's presidency.
Choi was an aide to the GNP's presidential hopeful Lee Hoi-chang
The Grand National Party (GNP) issued an apology after lawmaker Choi Don-woong said he had received 10bn won ($8.5m) from conglomerate SK
Group prior to presidential elections last year.
Prosecutors have already charged Choi Do-sul, a confidant and former aide to President Roh Moo-hyun, with taking hundreds of thousands of dollars from SK Group after Mr Roh won December's presidential poll.
Even though Mr Roh is not implicated in the scandal, his popularity has plummeted and he has called for a referendum on his rule.
Choi Don-woong served during the presidential election campaign as chief financial officer for the GNP's candidate Lee Hoi-chang.
He refused to say what he used the money for but prosecution authorities suspect it was used for electioneering.
"We sincerely apologise to the people for causing concern over
the acceptance of SK money, to which our lawmaker, Choi Don-woong,
has admitted," GNP head Choe Byung-yul said in a statement.
The slush fund scandal has now engulfed all three of South Korea's main political parties.
Also under interrogation is Lee Sang-soo, a member of a new reformist faction which recently broke away from the Millennium Democratic Party (MDP).
Mr Lee has admitted to receiving 2.5bn won from SK late last year
when he was MDP secretary general but maintains it was a legitimate political donation.
Push for referendum
President Roh is due to meet the leaders of four main political parties over the weekend to urge them to agree to his proposed referendum.
Each party has voiced opposition to the plan, concerned that it would be unconstitutional or a ploy to boost his support ahead of parliamentary elections in April.
The president is currently without a power base in the National Assembly. He split from his own party, the MDP, last month, after it was wracked by mass defections to other parties.
An MDP splinter group of Roh loyalists holds just 42 seats, compared to the 149-seat majority the GNP enjoys.
Mr Roh's popularity ratings have plummeted from more than 80% to below 30%.
He has been tarnished by political scandals, an economic recession, and tension over the South's relations with the North and the United States.
Polls suggest that Mr Roh would survive a referendum.