Serbia prepared for a "milestone" leap towards European Union membership Thursday seen as opening a new chapter in the troubled history of the Western Balkans.

A final verdict on Serbia's bid to join the EU club is due from leaders of the 27-nation bloc shortly after they gather from 1700 GMT for a two-day summit in Brussels.

The decision to grant Belgrade official EU candidate status, a first but crucial step in an often long and rocky road to full membership, requires a unanimous vote from the 27 member states.

Serbia's 2009 application, launched in the throes of the financial crisis and amid worries that the EU had expanded too far and too fast in its 2004 "big bang" enlargement, has been fraught with problems.

Seen finally as a shoo-in for membership after last year's arrest of war criminals Ratko Mladic and Goran Hadzic after almost two decades on the run, Belgrade suddenly was held back and told to do more for Balkans peace by defusing tension with breakaway Kosovo.

While staunchly refusing to recognise Kosovo's 2008 independence, it joined an EU-sponsored dialogue with its former province a year ago, aimed at smoothing relations and overcoming daily headaches caused by the border row—problems such as disrupted road, rail and telecommunications networks.

But Belgrade's hopes of a subsequent EU pat on the back at a December summit were dashed notably by Germany and Britain, among 22 EU nations to have recognised Kosovo. More was demanded from the Serbia-Kosovo dialogue.

"We encourage Serbia to build on that dialogue and to improve relations with Pristina for the sake of regional stability," EU president Herman Van Rompuy said at the time.

Other EU nations fretted over the friendship with Russia enjoyed by one of the last ex-communist states of Eastern Europe still outside the bloc.

But last week, Serbia and Kosovo, both keen to inch closer to the bloc, staged a key breakthrough.

In an 11th-hour deal just days before the EU summit deadline, Belgrade agreed to allow Kosovo to speak under its flag in regional meetings and to sign international agreements like any other fully recognised nation.

"It was an historic agreement," said EU foreign policy chief Catherine Ashton.

"Serbia deserves candidate status," added European Commission president Jose Manuel Barroso.

But earlier this week, Serbia again faced disappointment when EU ministers due to have given their prior approval to Belgrade's candidacy, were blocked by objections from Romania.

Despite anger from big powers such as Germany, Bucharest refused to budge on a demand for guarantees for Serbia's Romanian-speaking minority of Vlachs. It finally won a pledge from the EU executive to monitor minority rights in Serbia.

Kosovo for its part was granted an EU pledge for a feasibility study on striking a future trade and political accord with the EU.

"We have reached an important milestone," Kosovo President Hashim Thaci said Thursday.

For pro-European Serbian President Boris Tadic, largely credited with bringing the country closer to entry, Thursday's decision is a key to stopping nationalist victories in May elections in case of a new EU rebuff.

Of the six nations once part of the former Yugoslavia, Slovenia joined the EU in 2004 while Croatia is to be anointed the 28th member in 2013.

Macedonia and Montenegro are both official candidate countries but have yet to be approved to open formal adhesion negotiations, a process that can take years. Politically divided Bosnia has made no move towards the EU.

"The status of candidate will not bring us much in direct and fast benefits," said former premier Zoran Zivkovic this week. But "it is a historic event, strategically linking the future of small and poor Serbia with the huge European family."