Serbia's parliament has overwhelmingly rejected a United Nations proposal on the future status of Kosovo.
President Tadic said the plan violated international law
Lawmakers also backed a resolution stating the UN plan would illegally lay the foundations for creating a new state within Serbian territory.
Under proposals unveiled by UN envoy Martti Ahtisaari earlier this month, Kosovo would gain a form of self-rule which stops short of full independence.
Final talks on Kosovo's future will take place later this month.
The BBC's Nick Hawton says that there was never any doubt about how the vote would go, with Serbia's political establishment united against the proposals.
Acting Prime Minister Vojislav Kostunica said the plan would "dismember Serbia and grab 15% of its territory", while President Boris Tadic said it violated the UN charter "which guarantees inviolability of internationally-recognised states".
Under the plan, Kosovo would be able to join the UN and have its own flag and national anthem.
But the proposal stops short of the full independence from Serbia that its ethnic Albanian population - who comprise 90% of Kosovo's two million people - are seeking.
The UN has administered Kosovo since a Nato bombing campaign forced out Serbian troops in 1999.
A final round of UN-sponsored talks begins in Vienna on 21 February, but a compromise solution seems unlikely at this stage, our correspondent says.
The international community may be forced to impose a decision on the province's long term future, he says, something Serbia has said it would never accept.