Divisive internet personality Andrew Tate and his brother Tristan had an appeal against their detention rejected by a Romanian court on Monday.
Prosecutors are investigating the two brothers — who have been in police custody since December 29 — and two Romanian women for suspected human trafficking, rape and forming a criminal gang to sexually exploit women.
They have all denied the accusations.
The Tate brothers, who have dual US and British citizenship, have had their detention extended until the end of March.
Prosecutors can ask Romanian courts to extend suspects' detention for up to 180 days.
A court on February 21 ruled the two female suspects, Georgiana Naghel and Luana Radu, should be placed under house arrest when their current arrest warrant expires on Monday.
The brothers' lawyers had argued against their continued detention, saying prosecutors had not given new evidence or brought new charges.
Andrew Tate arrested in Romania over human trafficking case — video
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Judges have previously said the brothers posed a flight risk and their release could jeopardise the investigation.
Andrew Tate, a former professional kickboxer from Luton in southern England, has millions of followers online and he promotes a luxury lifestyle from wealth he says he earned from investments including a pornographic webcam business.
He gained mainstream notoriety for misogynistic remarks that got him banned from all major social media platforms, although Twitter reinstated his account after Elon Musk bought the company last year.