Carla del Ponte said she was pained to be stepping down after eight years as chief UN war crimes prosecutor with genocide suspects Radovan Karadzic and Ratko Mladic still at large.
The Swiss made a final appeal to the international community today to keep up pressure on Belgrade to hand over the Bosnian Serb wartime leader and Mladic, his military chief.
"The fact that Ratko Mladic and Radovan Karadzic are still at large is a stain on our work, a stain on all these great achievements," she told a news conference before leaving the job at the end of the month.
She said Mladic - who is believed to be hiding in Serbia - would never be arrested if the European Union signed an agreement allowing Serbia to advance in its membership bid.
"My biggest fear today is political issues are taking priority over international justice," she said in an allusion to mounting pressure to compensate Serbia for the expected declaration of independence by its province of Kosovo next year.
Ms Del Ponte said she was confident her successor, Belgian Serge Brammertz, would energetically pursue Mladic, Karadzic and the two other ethnic Serb indictees still at large.
The Hague tribunal, which has sentenced 53 and still has proceedings ongoing against 50 accused, is due to wrap up its work in the next couple of years.
"The tribunal must not close its doors until all remaining fugitives are brought to justice," Ms Del Ponte said.
The former Swiss attorney general, who made her name chasing mafiosi and money launderers, said she had drawn much of her energy from the victims of the crimes committed during the wars that tore apart the former Yugoslavia in the 1990s.