The chief judge at The Hague says the trial of ex-Yugoslav President Slobodan Milosevic needs a "radical review" because of the defendant's poor health.
Milosevic has waited two years to start his defence
Judge Patrick Robinson issued his warning after a court-appointed doctor's report said Mr Milosevic needed to rest.
The judges delayed the start of Mr Milosevic's defence case and said they would decide later how to proceed.
Mr Milosevic has been on trial for genocide and war crimes since 2002.
Judge Robinson quoted a cardiologist's report saying the 62-year-old former leader had high blood pressure and was at risk of a heart attack.
"It is therefore necessary to navigate constantly between sufficient rest, optimum medication and the stress of the trial," the judge read.
Mr Milosevic's health problems have led to frequent delays in the trial, seen as the most important war crimes trial in Europe since the Nuremberg trials of Nazi leaders following World War II.
"The time has come for a radical review of the trial process," Judge Robinson told the court.
Steven Kay, a lawyer appointed by the court to ensure that Mr Milosevic gets a fair trial, said the future of the trial was now in doubt.
"It's quite clear over the past five months that [Milosevic's] health had been gradually declining.
"It may well be that the court is at a stage now of having to consider that as a distinct issue on this trial as to his very fitness to stand trial at all."
The BBC's Chris Morris at The Hague says this is the first time that a former head of state has been forced to stand trial in an international court.
But, he says, after more than two years, legitimate questions are now being raised about whether it can ever be brought to a fair conclusion.
A flushed-looking Mr Milosevic berated the judges for making him come to court on Monday.
"My health situation has deteriorated as a direct result of your refusal to enable me to get my health back," he said.
Began Feb 2002
Milosevic facing more than 60 charges of war crimes
Court heard from 295 witnesses
Prosecutors called for a defence lawyer to be assigned to Mr Milosevic, who is representing himself - a suggestion that angered the former leader.
Mr Milosevic was due to defend himself against charges of genocide, crimes against humanity and war crimes committed in Bosnia, Croatia and Kosovo in the 1990s.
The accusations include "genocide or complicity in genocide" at Srebrenica, where more than 7,000 people were killed in 1995.
Mr Milosevic has waited two years for the chance to give his version of events, after listening to evidence against him.
He was expected to ask to call key leaders to give evidence, including UK Prime Minister Tony Blair and former US President Bill Clinton.
Prosecutors finished their case in February, but BBC legal affairs analyst Jon Silverman says it did not deliver the spectacular knockout punch which it promised before the trial began.
Mr Milosevic's witness list - at about 1,600 - is more than five times longer than the number called in two years by the prosecution.
Mr Clinton and Mr Blair were both key leaders during Nato's bombing of the Yugoslav capital Belgrade during the 1999 Kosovo conflict.
Also on the list, lawyers say, are former US Secretary of State Madeleine Albright, who was a US ambassador to the UN during the Bosnian war, and former UK Foreign Secretary Robin Cook.