By Ivana Sekularac

BELGRADE, Sept 6 (Reuters) - The leader of Serbia's main opposition party, the nationalist Radicals, resigned on Saturday because the party leadership would not back Serbia's bid to join the European Union, the state news agency Tanjug reported.

Tanjug quoted Tomislav Nikolic as saying the party's top leadership had decided not to vote for ratification of the Stabilization and Association Agreement with the EU.

Nationalist parties oppose the pact on the ground that implementing it would amount to consenting to the secession of Kosovo -- a step supported by a majority of the 27 EU members.

Serbia lost control of the Albanian-majority province of Kosovo in 1999, when NATO bombs drove out Serb forces accused of mass killings of civilians in a two-year guerrilla insurgency.

When Kosovo declared independence in February, the Serbian government fell because of disagreements over whether it should still try to join the European Union.

Nikolic, a moderate nationalist, said on Thursday that his party would vote for the agreement if a provision stating that Kosovo was an integral part of Serbia was added to the ratification document.

His party's top executives rejected the agreement outright, triggering Nikolic's resignation as party leader. "I am going to stay a party member if it is accepted by the board," he said.

Serbian media reported earlier on Saturday that the Radical Party had split into two factions, one led by Nikolic and the other by Vojislav Seselj, the party founder who is being tried for war crimes in The Hague.

"This split is going to weaken the Radical Party for sure," said analyst Dragan Bujosevic. "The party is never going to achieve the results it did with Tomislav Nikolic."

In January Nikolic narrowly lost the presidential race to pro-Western Boris Tadic, who has tried to accelerate Serbian progress towards joining the European Union.

Nikolic has led the Radicals since 2003, and has tried to change the party's image from that of the 1990s, when its members fought alongside Serb paramilitaries in wars in Croatia and Bosnia that marked the breakup of Yugoslavia.

The Radicals won 30 percent of the vote in general elections on May 11, but were unable to form a nationalist coalition government because the Socialists shifted to the pro-Western bloc and embraced the goal of EU membership.

The European Union has said Serbian membership of the bloc depends on the arrest and extradition to the war crimes tribunal in The Hague of two leading war crimes fugitives.

In July, Serbia arrested Bosnian Serb wartime leader Radovan Karadzic after 11 years on the run, and he is now on trial in The Hague. Serbian officials say they are still searching for Karadzic's wartime military commander, Ratko Mladic. (Editing by Tim Pearce)

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