By Matt Prodger
BBC News, Belgrade
Montenegro's parliament has unanimously agreed to hold a referendum on independence from Serbia on 21 May.
Montenegro's parliament overwhelmingly backed the referendum
The agreement, brokered by the European Union, follows months of wrangling between the pro-independence governing coalition and the opposition.
The union of Serbia and Montenegro is all that remains of the federation of six republics which made up Yugoslavia before the wars of the 1990s.
Montenegro's prime minister wants to cut official ties with Serbia.
Now Montenegrins will decide on 21 May if they want to separate from Serbia.
Montenegro's long-serving Prime Minister, Milo Djukanovic, says the much bigger partner in the union is holding his country back.
He says he wants to free Montenegro from Serbia's problems, like the unresolved issue of Kosovo and the failure to hand over war crimes fugitives to the Hague tribunal.
But the referendum raises other problems.
The two countries have traditionally been very close and their populations are somewhat mixed up.
Many Montenegrins consider themselves Serbs while, across the border, many Serbs boast of Montenegrin ancestry.
Both sides in the debate have been accused of exacerbating ethnic divisions.
And given recent history in the Balkans the European Union is keen to have the referendum conducted in as calm an atmosphere as possible.