On Friday, the Seoul Central District Court dismissed the prosecution's initial request for a 10-day extension, citing a lack of reasonable grounds for the prosecution to continue the investigation.
Prosecutors filed their second request on Saturday, only to be denied on similar grounds.
Prosecutors must indict him before the detention period expires on Monday or release him.
The Corruption Investigation Office for High-ranking Officials (CIO) transferred the case to the prosecution on Thursday after several failed attempts to question Yoon face-to-face following his formal arrest last Sunday.
The CIO is not authorized to indict suspects.
The court said Friday that the CIO was established to ensure fair and independent inquiries by separating investigation and prosecution powers, providing no legal grounds for prosecutors to further investigate a case transferred by the CIO.
Yoon, who is currently being held in solitary confinement at a detention center near Seoul, is also facing an impeachment trial at the Constitutional Court.
The Constitutional Court has been deliberating since mid-December on whether to oust or reinstate him, following the National Assembly's vote to impeach him over his declaration of martial law on Dec. 3.
Copyright ⓒ Aju Press All rights reserved.