"Those who signed the tabilization and Association Agreement [SAA] are fully responsible for it and they signed it in their own names and never in the name of Serbia," Kostunica said in an interview with the daily Vecernje Novosti.
Serbian President Boris Tadic and Deputy Prime Minister Bozidar Djelic signed the SAA, which legalizes the secession of Serbia's province of Kosovo and is a precursor to EU membership talks, with the European Union on Tuesday, a move denounced by Serbian nationalists as treason.
The nationalist majority in Serbia heaped insults on the SAA with the European Union, calling its signing "lies and deceit," "treason," and "legally null and void."
Kostunica's Democratic Party of Serbia [DSS] has said it will take legal action against the pro-Europe bloc for signing the SAA, while the ultra-nationalist opposition Serbian Radical Party [SRS] has said it will launch a case to remove Tadic from office.
"Without an SAA there can be no possibility for a full integration of the Western Balkans into the European Union, and without such integration there is no possibility for a final reconciliation amongst the people of the Western Balkans," Tadic said.
Observers hope that the SAA will boost the chances of the pro- European bloc, led by Tadic, in snap parliamentary elections on May 11.
Rising nationalists challenge pro-Europe parties
The latest surveys published Wednesday showed that the anti- European bloc, led by the increasingly nationalist Kostunica and ultra-nationalist opposition Radical Party, could win more seats in parliament than the pro-European parties.
The Medium Gallup and Blic daily poll showed that the nationalists would win 137 seats out of 250 if elections were held right now, while the pro-European bloc would win 113 seats.
If Kostunica forms a government with the Radicals, that would cement Serbia's turn away from the EU towards Russia. The alternative would be for Kostunica to form a coalition with Tadic, as he did last year.
Observers say that, either way, the process of forming a government will be long and very difficult followed by months of negotiations, political trading and blackmail.
A door to EU membership and stability
Kosovo declared independence from Serbia in February. The youngest European state was recognized by most EU member states and major Western powers. Serbia, backed by its ally Russia, opposes it.
The SAA is a document setting out how an aspiring EU member should reform its political, economic and legal systems to come in line with EU norms, and how the EU should help it do so.
As its name suggests, it is designed to bring basic stability and prosperity to a region devastated by the conflict that erupted after the implosion of the former Yugoslavia.
The ratification of Serbia's SAA was made conditional on Belgrade proving that it was cooperating fully with the International Criminal Tribunal for the former Yugoslavia.