If Mr Nikolic were to win this election, he could be expected to share Prime Minister Vojislav Kostunica's view that any EU mission in Kosovo without UN endorsement would be illegal and everything should be done to make life difficult for it.

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In contrast Mr Kostunica has, in effect, said that the possibility of co-operation ended once the EU began deploying a big mission in Kosovo.

ECONOMIST: Violence in the capital of Serbia

In a report the Commission said Serbia had "actively and constructively" engaged in dialogue with Kosovo and had improved its co-operation with Eulex, the EU rule-of-law mission in Kosovo.

BBC: EU Commission: 'Start Serbia membership talks'