For the second time in a fortnight Park rejected a court order to give evidence in the case of Lee Jae-Yong, the vice- chairman of Samsung Electronics.
Lee is accused of bribery, corruption, perjury and other offences stemming from a wide-ranging scandal that saw Park herself impeached and arrested. Four other Samsung executives have also been charged.
Park, now in the Seoul detention centre and on trial separately, has been complaining of exhaustion and a toe injury that has left her limping slightly.
"We visited the accused in the Seoul detention centre this morning to have her testify in court but she, again citing health reasons, refused to comply with the summons," a prosecutor was quoted as saying by Yonhap news agency.
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Senior Judge Kim Jin-Dong said Tuesday he planned to finish Lee's trial next month. He denies the charges.
Prosecutors will make their sentence demand on August 4 and a verdict is likely two weeks after that, a court spokesman told AFP.
Choi, who allegedly used some of the funds for personal gain, was jailed for three years last month for obstruction of business in connection with her daughter's illicit admission to a Seoul university. She also faces other cases.
Samsung, the world's biggest smartphone maker and South Korea's largest business group with revenues equivalent to about a fifth of the country's GDP, was the single biggest donor to the foundations.
On Wednesday Lee, wearing a dark suit with no tie, watched quietly as prosecutors and defence lawyers questioned a former presidential Blue House official who handed prosecutors the notebooks of a former top presidential aide.
Prosecutors argue that the notebooks, in which aide Ahn Jong-Beom jotted down Park's instructions, constituted incriminating evidence against Park, Choi and Lee.
Ahn has been arrested on charges of abuse of power and attempted coercion in helping Choi set up the foundations.
Lee's arrest sent shockwaves through the group and triggered the announcement of a major reform of its top-down management style.
The groups have faced increasing criticism over their management practices, including rapid promotions for family members -- some of whom have damaged the corporate image with their conduct.