The war crimes trial of ex-Bosnian Serb commander Ratko Mladic has been postponed because prosecutors failed to disclose some evidence to the defence.
About 7,000 pages of evidence, which should have been passed to Ratko Mladic's lawyers, were not handed over.
The defence has asked for six months to process that material.
A BBC correspondent says it appears to have been a clerical error, but the presiding judge is under great pressure to grant a very significant delay.
It has been an embarrassing day for international justice, says the BBC's Allan Little in The Hague.
Ratko Mladic is facing 11 charges of war crimes and crimes against humanity over the 1992-1995 Bosnian conflict. He denies all of them.
Presiding judge Alphons Orie said judges would analyse the "scope and full impact" of the error and aim to establish a new starting date "as soon as possible".
The presentation of evidence in court was supposed to begin towards the end of the month.
The halt came on the second day of the trial after prosecutors submitted evidence alleging that Mr Mladic orchestrated the massacre of more than 7,000 Muslim men and boys at the Bosnian town of Srebrenica in 1995.
Much of the material that has not been passed to the defence focuses on witnesses who prosecutors had intended to call to testify before the court takes a three-week summer break beginning in July, court spokeswoman Nerma Jelacic told the Associated Press news agency.
Prosecutors had already acknowledged the errors and did not object to the delay.
The tribunal published a letter on Thursday from prosecutors to Mr Mladic's lawyer which says the missing documents were not uploaded onto an electronic database accessible to defence lawyers.
"We sincerely apologise for the inconvenience that these missing materials... may have caused to you," the letter dated 11 May says.
Earlier, prosecutors described the chaos leading up to the 1995 Srebrenica massacre.
Video shown to the court in The Hague showed panicking civilians swamping UN trucks as Bosnian Serb forces neared.
Prosecuting counsel Peter McCloskey said the crimes at Srebrenica had never been in dispute so the prosecution's focus would be on individual criminal responsibility.
He said the Bosnian Serb Army was not an "army out of control" and that Gen Mladic had been on the ground and in command.
The court watched video of local people panicking in a UN compound outside Srebrenica on 11 July 1995 as Bosnian Serb forces approached, followed by scenes of Gen Mladic triumphantly entering the town.
Speaking directly into the camera he says: "We give this town to the Serb nation as a gift. The time has come to take revenge on the Muslims."
Gen Mladic has presented an alibi for a crucial period of three days that followed. But the prosecution say they do not accept that he was unaware of what was happening.
The Srebrenica massacre was the worst atrocity in Europe since the end of World War II.
Serb fighters overran the enclave in eastern Bosnia - supposedly under the protection of Dutch UN peacekeepers. Bosniak (Bosnian Muslim) men and boys were separated off, shot dead and bulldozed into mass graves - later to be dug up and reburied in more remote spots.
Gen Mladic is also charged in connection with the 44-month siege of Sarajevo during which more than 10,000 people died.