The International Criminal Tribunal for the former Yugoslavia (ICTY) "orders the trial chamber to immediately revoke Seselj's provisional release and order his return to the United Nations Detention Unit," in The Hague, court documents said.
The tribunal in November allowed Seselj to return home to Serbia for cancer treatment pending an ICTY verdict on alleged war crimes during the Balkan wars.
Since his release, 60-year-old Seselj has repeatedly lashed out at the tribunal, vowing not to return for his sentencing or to serve any time, as well as resuming his poisonous nationalist rhetoric.
"I'm certainly not going to return to The Hague of my own free will," Seselj told AFP by telephone in Belgrade following today's decision, adding that police had yet to contact him.
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"I was expecting the 'cavalry' to arrive with a lot of fuss to arrest me, but they haven't yet turned up," he said.
Seselj, accused of leading ethnic Serb volunteers in persecuting Croats, Muslims and other non-Serbs during the 1990s wars, underwent colon cancer surgery in 2013.
Serbia said it would allow the return of the ultra-nationalist firebrand, who was repeatedly found guilty of contempt during his trial.
The prosecution appealed against his provisional release, saying his public statements called into question the tribunal's assessment of his health and that his "threats to persons who cooperated with the prosecution breached the condition of his provisional release not to obstruct the course of justice."
Seselj has pleaded not guilty to nine counts, including murder, torture, cruel treatment and wanton destruction of villages.
Prosecutors said the radical politician recruited and indoctrinated volunteers and paramilitaries, known as "Seselj's men" who committed atrocities during the Balkan conflicts.