Ousted South Korean president Park Geun-hye has been formally charged in a corruption scandal that led to her impeachment.
Prosecutors say the charges she faces include bribery, coercion, abuse of power and leaking state secrets.
Both women deny the charges. Ms Park was removed from office last month.
She lost her presidential immunity and was dismissed from her post when the constitutional court upheld a decision by parliament in December to impeach her.
Ms Choi is accused of using her presidential connections to put pressure on companies to give millions of dollars in donations to non-profit foundations she controlled.
Ms Park is alleged to have been personally involved in this, and to have given Ms Choi unacceptable levels of access to official documents.
Those companies include the country’s largest group, Samsung. Its acting head Lee Jae-yong and Ms Choi are in the same detention centre as Ms Park awaiting trial.
Lotte Group chairman Shin Dong-bin was also charged with bribery on Monday, but prosecutors did not detain him. Both Samsung and Lotte deny wrongdoing.
Hwang Kyo-ahn, who is loyal to Ms Park, is now the acting president and an election is to be held by 9 May.