President Yoon Suk Yeol on Friday exercised his veto power to strike down a bill that would let the opposition-controlled National Assembly order a special investigation into first lady Kim Keon Hee's alleged involvement in stock manipulation.

"(We) express a deep regret over the opposition bloc's unilateral move (to pass the bill to investigate the first lady) without bipartisan consent," presidential chief of staff Lee Kwan-sup told reporters. "Yoon has the obligation to veto the bill that goes against constitutional principles."

Yoon's veto came less than an hour after an extraordinary Cabinet meeting Friday, presided over by Prime Minister Han Duck-soo, to ask Yoon to veto the bill that was sent to the government Thursday afternoon.

Han said the newly nominated special counsel as a result of the special bill's passage is "unlikely to maintain political neutrality and conduct a fair investigation."

The opposition bloc, comprising four parties led by the main opposition Democratic Party of Korea, blasted Yoon's decision as nearly 180 lawmakers gathered in front of the National Assembly.

Rep. Hong Ihk-pyo, floor leader of the Democratic Party, said the president was abusing the power of his Cabinet "to protect Kim and himself like a bulletproof vest."

Hong also said Yoon's decision neglected the voices of the people, citing multiple polls where over 3 in 5 responded that Yoon should not exercise his veto power in the case.

Since last month, Hong has said that the bloc could consider filing a court appeal of the presidential veto, as one of the options available is to file an adjudication with the Constitutional Court to decide whether Yoon's exercise of the veto infringes on their power as granted by the Constitution.

The latest developments up the ante, as the opposition bloc has long expressed frustration over the slow pace of investigations by law enforcement and prosecutors into Kim's alleged involvement in white-collar crime dating back a decade, whereas others found to have been involved are already serving jail time.

The Democratic Party initially proposed the bill in April 2023 to circumvent the process and swiftly bring Kim to trial. The opposition unilaterally passed the bill in December with all 180 lawmakers of the bloc present voting in favor. The ruling People Power Party, holding 112 seats, boycotted the vote.

Under the bill, the opposition bloc would be granted the power to recommend special counsel candidates, and Yoon would pick one among them as the special counsel who would investigate his wife.

Amid such uncertainties, Kim has kept herself out of the public eye over the last three weeks. She was last seen immediately after her return from Yoon's state visit to the Netherlands in mid-December.

The opposition bloc believes that Kim may have masterminded the price manipulation of imported car distributor Deutsch Motors, a low-volume stock, in the early 2010s. South Korean courts found former Deutsch Motors Chairman Kwon Oh-soo and asset managers who managed Kim's money guilty of related crimes. Kim and the president tied the knot in March 2012.

Yoon's office and the ruling People Power Party both condemned the bill, saying the opposition bloc intends to manipulate public opinion by shaming the first lady in the run-up to the general election that will determine control of the National Assembly in three months.

The special bill was one of two passed by the opposition in December. The other is aimed at speeding up the investigation into a bribery case concerning a high-profile land corruption scandal in Seongnam, Gyeonggi Province. The opposition bloc will also be given the power to recommend special counsel candidates to look into who was allegedly bribed by Kim Man-bae, a key figure in the lucrative Daejang-dong land development project.

The bill over the bribery scandal was also vetoed by Yoon. Han said the nomination of the special counsel could "stand in the way of the prosecution's ongoing investigation into the matter."