Lawyers for Slobodan Milosevic could ask the Hague tribunal to subpoena witnesses after many refused to testify because the former Yugoslav president was not allowed conduct his own defence.
Court-assigned lawyer Mr Steven Kay said most of 150 defence witnesses contacted so far, from a list of more than 1,600 witnesses drawn up by Mr Milosevic, had refused to appear.
"It is clear to us now that a very large number of these witnesses are refusing to testify because they are against the court decision to appoint a defence counsel," Mr Kay said.
The trial's three judges ruled last month the 63-year-old, who suffers from high blood pressure and has a heart condition, was no longer fit to run his own defence.
But Milosevic, who defended himself for the first 30 months of the trial, has repeatedly challenged the decision, and refused to instruct the court-assigned defence lawyers, meet them or ask follow-up questions in court.
He originally planned to call all witnesses and lead the cross examination himself. He does not recognise the UN tribunal and declined to plead to charges against him. Not guilty pleas were entered on his behalf.
He is charged with genocide, crimes against humanity and war crimes in Croatia, Bosnia and Kosovo in the 1990s.