Supporters of South Korea's former president Park Geun-hye shout slogans during a protest demanding her release from prison, outside the Supreme Court in Seoul on Thursday. Photo: AFP

South Korea's top court ordered new trials Thursday for former president Park Geun-hye and Samsung heir Lee Jae-yong over the corruption scandal that brought her down, in a blow to the world's biggest smartphone and memory chip maker.

Park is serving a 25-year jail term after being convicted last year on bribery and abuse of power charges. 

But Lee, whose sprawling conglomerate is vital to the health of the world's 11th-largest economy, had many of his convictions quashed on appeal in February 2018 and was released after being given a suspended sentence.

Their trials highlighted shady links between big business and politics in South Korea, with Park and her close friend Choi Soon-sil accused of taking bribes from corporate bigwigs in exchange for preferential treatment.

South Korea's Supreme Court on Thursday sent all three of their cases back for new proceedings, saying that errors had been made in the judgments.

After a 10-month trial - in which she boycotted most of the proceedings in protest at being held in custody - Park was convicted in April last year of receiving or demanding more than $20 million from conglomerates, sharing secret state documents, "blacklisting" artists critical of her policies, and firing officials who resisted her abuses of power.

She was sentenced to 24 years, later extended for an additional 12 months.

But the Supreme Court ruled that under the country's public official election act, courts must rule separately on bribery accusations when incumbent or former presidents face multiple criminal charges.

"We send the case back to the Seoul High Court," said chief justice Kim Myeong-su.

South Korean media warned that the ruling might not work out in her favor, as if she is convicted again in two separate verdicts she could face a longer total sentence in total.

Samsung is by far the biggest of the family-controlled conglomerates that dominate business in South Korea, and crucial to the country's financial health.

Lee is vice chairman of its flagship subsidiary Samsung Electronics, and his case centerd on millions of dollars the group paid Choi, allegedly for government favors such as ensuring a smooth transition for him to succeed his ailing father.

He was initially jailed for five years in 2017 before his successful appeal, which chief justice Kim said "misunderstood the law on bribery."