The war crimes tribunal in The Hague has threatened to use subpoenas to force witnesses to testify at Slobodan Milosevic's trial.
Milosevic has dismissed the trial as a kangaroo court
A lawyer for the former Yugoslav leader says a proper defence is difficult, as many people are refusing to appear.
The lawyer, Stephen Kay, said they opposed the court's decision to impose defence lawyers on Mr Milosevic.
He is accused of committing genocide, crimes against humanity, and war crimes during the Balkans wars of the 1990s.
Mr Kay told the court that 80% to 90% of the witnesses he had approached would not testify.
Some were awaiting permission from their governments, he said.
Began February 2002
Milosevic faces more than 60 charges
Prosecutors' case rested February 2004
Court already heard from 295 witnesses
But many opposed the decision by the tribunal to impose lawyers on the defendant, he added.
Presiding Judge Patrick Robinson urged the witnesses to reconsider their opposition.
"The chamber will issue a subpoena because it has to be demonstrated to witnesses that this trial is a matter of fundamental importance," he told the court on Monday.
Mr Milosevic defended himself for 30 months - until the tribunal appointed two British defence lawyers in September.
Mr Kay told the International War Crimes Tribunal for the Former Yugoslavia that the number of defence witnesses willing to testify was at a critical low, and that this threatened to undermine the defence case.
Some witnesses may reconsider after an appeals court rules on whether to allow Mr Milosevic to represent himself, he added.
An appeal hearing is set for next Thursday.
But the BBC's Geraldine Coughlan in The Hague says it is not clear when the court will deliver its ruling.
Mr Kay said he had asked foreign governments to make the witnesses available, and that he would consider asking the court to subpoena about 20 witnesses, including state officials, if the requests failed.