South Korea's Supreme Court overturned on Thursday an earlier ruling that had cleared election frontrunner Lee Jae-myung of election law violation, throwing into doubt his eligibility to run for the presidency.

Prime Minister Han Duck-soo has been mulling whether to run for election, while the main opposition candidate and front-runner suffered a setback in court.

Han Duck-soo has emerged as a potential conservative standard bearer as the main conservative People Power Party remains in disarray.