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 contest the election, while the main opposition candidate and front-runner suffered a setback in court.

South Korea is holding an early presidential election on June 3 after conservative President Yoon Suk Yeol was ousted over his imposition of martial law. Han had been appointed prime minister, the No. 2 post, by Yoon, and he served as acting leader after Yoon was suspended and later removed from office.