Carla Del Ponte, the chief prosecutor, said on Friday that a plan was urgently needed to ensure that trials could continue.
At a meeting of international war crimes prosecutors in the Hague she said political pressure to wrap up proceedings was likely to make way for financial pressure next year.
The UN funds the court and can cut back its multimillion-dollar budget, possibly urging member states to make voluntary contributions outside the UN budget to keep the court running.
The UN tribunal is also being pressured by the UN Security Council to wrap up all trials by 2008 and to finish work on appeals two years later. But major suspects are still free.
Radovan Karadzic, the former Bosnian Serb leader, and Ratko Mladic, the alleged architect of the murder of about 7500 Muslims in Srebrenica in 1995, are still on the run.
Those charged over Srebrenica
More than two dozen people are on trial at the tribunal in cases expected to last months, if not years.
Del Ponte said: "The completion strategy was never intended to foster impunity for those accused of genocide and other most serious crimes.
"If we do not devise today substantial measures for the task of the ICTY to be completed, this is exactly what will happen.
"I trust the international community cannot tolerate such a blatant breach of its principles and commitments. The victims of the Balkan wars do not deserve to be betrayed once more."
She said having Karadzic and Mladic stand trial in either Serbia or Bosnia, where they are regarded as heroes by some, instead of the Hague would be "absolutely unimaginable".
Mladic and Karadzic are believed to be hiding in Serbia.
Officials in Belgrade, under pressure from the EU to hand over war crimes suspects, say they are doing all they can to arrest them.
Del Ponte said her prosecutors were working with tribunal judges and defence attorneys to streamline trials.
She said that without their own police force, catching fugitives remained a problem.