South Korea's Prime Minister Han Duck-soo has been reinstated as acting president after the country's Constitutional Court dismissed a motion for his impeachment 87 days after the motion was filed.

The motion was dismissed in a five-to-one vote by the court's eight justices on Monday.

The ruling overturned the National Assembly's motion to impeach Han on Dec 27, less than two weeks after he took over from President Yoon Suk-yeol, who was impeached following his short-lived imposition of martial law on Dec 3.

The five justices who voted to dismiss the motion said there was a lack of evidence to support the claim that Han violated the Constitution or the law concerning martial law, a court statement said.

One justice voted to uphold Han's impeachment, and two others said the impeachment motion was invalid because the quorum in the assembly did not meet the required two-thirds majority, or 200 votes, that applies when a president is impeached.

The opposition-led parliament submitted the motion after Han refused to appoint three more justices to the Constitutional Court that will preside over Yoon's impeachment trial.

The nine-member bench now has only eight justices because one has yet to be appointed by the country's acting president.

Though Han's refusal to appoint Constitutional Court justices was deemed unconstitutional, the court statement said there was a lack of evidence to suggest it "was motivated by an intent or purpose to incapacitate the Constitutional Court "in Yoon's impeachment trial.

Therefore, "it could not be concluded that the respondent had betrayed the trust of the people" to remove him from office, the court said.

With the court's ruling taking effect immediately, Han was directly reinstated as prime minister and acting president.

In a televised speech, Han vowed to protect South Korea's interests amid a global trade war initiated by the US. A deeply divided society can satisfy no one, Han said, and the government and the political parties need to change.

"I sincerely ask for bipartisan cooperation between the ruling and opposition parties so that the Republic of Korea can once again leap forward and overcome the current crisis."

The National Assembly Speaker Woo Won-sik said he respected the Constitutional Court's decision on Han's impeachment trial and called for Han to immediately appoint the ninth justice, Yonhap News Agency reported.

'Stern warning'

The ruling People Power Party said the verdict is "a stern warning from the judiciary against excessive legislative violence by the opposition party", which holds a majority in the assembly.

The interim leader of the PPP, Kwon Young-se, said he believes the ruling over Han's impeachment case will not have a direct impact on Yoon's impeachment case as the two are separate issues.

Park Chan-dae, floor leader of the main opposition Democratic Party, said the Constitutional Court should immediately deliver its ruling on Yoon's impeachment trial.

The ruling on Han's impeachment marked the beginning of a critical week in South Korean politics.

An appeal by the main opposition leader Lee Jae-myung against his conviction for election law violations, which will determine his eligibility for future presidential elections, is set for a verdict on Wednesday.

Legal analysts said the verdict on Yoon's impeachment would closely follow Han's case and could be delivered as early as Friday.

The Constitutional Court has 180 days from Dec 14, the day it received Yoon's impeachment case, to decide whether to remove Yoon from office or restore his presidential powers.

If he is impeached, a presidential election must be held within 60 days of his exit. If the impeachment motion is dismissed, Yoon will be reinstated.

Yoon also faces a separate criminal trial on charges of leading an insurrection by declaring martial law.

The Seoul Central District Court on Monday held the second preparatory hearing of Yoon's criminal trial in Yoon's absence. The first formal hearing was scheduled for April 14.

Xinhua contributed to this story.

kelly@chinadailyapac.com