Rwandan genocide suspect Felicien Kabuga should be urgently considered for release after he was declared unfit to stand trial in The Hague, UN appeals judges ruled on Monday.
Former tycoon Kabuga, who is 88 according to officials but claims to be 90, is accused of setting up a hate broadcaster that fuelled the 1994 slaughter of around 800,000 people.
A war crimes tribunal said in June that Kabuga was no longer healthy enough to go on trial, but that he should still undergo a stripped down legal process without a verdict.
However appeals judges said on Monday that the lower court made an "error of law" and that there was no legal basis for an "alternative finding procedure" instead of a trial for Kabuga.
They had "decided to remand the matter to the trial chamber with an instruction to impose an indefinite stay of proceedings in view of Mr Kabuga's lack of fitness to stand trial."
"The appeals chamber further instructed the trial chamber to expeditiously consider the issue of Mr Kabuga's detention on remand," the judges said in a ruling.
The decision "must be disappointing" to victims and survivors of the 1994 slaughter who have "waited long to see justice delivered," the appeals judges acknowledged.
But they said that "justice can be delivered only by holding trials that are fair and conducted with full respect for the rights of the accused".
Captured in Paris 2020 after two decades on the run, wheelchair-bound Kabuga went on trial last September and pleaded not guilty.
Prosecutors accuse Kabuga, once one of Rwanda's richest men, of being the driving force behind Radio-Television Libre des Mille Collines (RTLM), which urged ethnic Hutus to kill Tutsis with machetes.
But judges said in June that medical experts had now found that he has "severe dementia".
The court first put the trial on hold in March over health concerns, having earlier dismissed bids by Kabuga's defence lawyers to have him declared unfit to stand trial.