Lawyers for eight men accused of taking part in the 1995 Srebrenica massacre have demanded the replacement of the three-judge panel at their trial in Serbia.
The trial was due to start on December 12 at a war crimes court in Belgrade, but it was adjourned after defense lawyers made the motion.
It is the first time Serbia will try suspects involved in the massacre of some 8,000 Muslim men and boys from the Bosnian town of Srebrenica during the Bosnian War.
The eight are facing charges of ordering or taking part in the killing of hundreds of Muslims in the town of Kravica, near Srebrenica.
One of the defense lawyers, Goran Petronijevic, said the judges had violated an obligation to reveal the identity of protected witnesses to the defense team.
"Today's hearing did not begin because the judge panel was in a flagrant violation of the criminal proceedings law," Petronijevic said outside the courthouse. "Fifteen days before the start of the hearing, they were obliged to reveal the identity of the protected witnesses to the defense team."
A ruling on the request for different judges to be appointed is expected by December 13.
The defendants are suspected of having been members of a Bosnian Serb special police unit called Jahorina.
The group includes the unit's commander, Nedeljko Milidragovic, who is accused of having ordered the executions.
If found guilty, they face up to 20 years in jail.
Former Bosnian Serb military commander Ratko Mladic, who is accused of helping orchestrate the Srebrenica massacre, is currently on trial in The Hague.