North Korea has denied sending spies into South Korea to murder the most senior official ever to defect from the Communist country.
Seoul said last month it had arrested two men on suspicion of being on a mission to kill Hwang Jang-yop, 87.
But Pyongyang said the accusation was a "ridiculous fabrication", its state-run Uriminzokkiri website reported.
Mr Hwang was once a close confidant of North Korean leader Kim Jong-il but defected to the South in 1997.
The alleged plot to kill him was uncovered when the two men travelled to South Korea from Thailand earlier this year, posing as defectors themselves, said South Korea's Yonhap news agency.
During the debrief that all defectors go through, they reportedly told police they had orders to "slit the betrayer's throat".
The news came two weeks after an apparent death threat against Mr Hwang appeared in Uriminzokkiri warning that "traitors have always been slaughtered with knives".
But on Wednesday, the website said South Korea had fabricated the assassination story to boost anti-North sentiments.
Tensions are high between the two countries as investigations continue into what sank a South Korean warship in March, with the loss of 46 lives.
Officials say the Cheonan was sunk by an underwater explosion, but have been careful to avoid pointing the finger at North Korea.
However, earlier this week, South Korean President Lee Myung-bak described the incident as "a grave international issue involving inter-Korean relations".
North and South Korea are still technically at war, since the 1950-53 conflict ended without a peace treaty.
Mr Hwang lives under heavy police protection at an undisclosed location in South Korea and has remained a harsh critic of Pyongyang.