Speaking at a Christmas reception organised by the Serb National Council on Friday, Sanader said the Croatian Foreign Ministry would send Slovenia a harsh protest note urging it to finally agree to have the matter settled before an international judicial body.
He stressed that the middle line in Savudrija Bay would remain the borderline at sea and that anyone sailing in Croatia's territorial waters would have to respect Croatian laws.
Asked whether he believed that Croatia-Slovenia relations were at the lowest level so far, Sanader said that this was not about bilateral relations, but about another proof that the issue must be solved before an international judicial body.
He added that unilateral moves only bore witness to "nervousness" and stressed that Croatia would present strong arguments before an international judicial body.
"Some in Slovenia are afraid of that," he added.
Asked whether Croatia would follow Slovenia's example and report the neighbouring country to European Union institutions, Sanader replied: "We are no tattlers, but we will know how to protect our interests".
He said that Croatia yesterday informed the European Commission about the Slovene government's move and recalled that European Enlargement Commissioner Olli Rehn had confirmed at his last meeting with Croatian authorities on December 19 that the issue was not a European but bilateral issue.
Social Democratic Party leader Ivica Racan, who chairs the National Committee monitoring accession talks with the EU, said Slovenia could not cite the agreement which he and former Slovene prime minister Janez Drnovsek had initialled in 2002, because the Croatian parliament did not ratify the agreement and it did not go into force.
He recalled that he had informed Drnovsek at the time of the failure to solve outstanding bilateral issues completely.
Racan proposed that the two countries try to continue negotiations, without blackmailing, or go to international arbitration.
Responsibility for that rests with Slovene politicians, he added.
"I have always been aware of Slovenia's ambitions. Their frustrations are not likely to be directed towards the North or West, it is easier to direct them towards the East and towards Croatia," the SDP leader said.
He recalled that the EU included countries with outstanding border disputes, but "those countries are not dramatising those problems, so one should ask oneself why the Slovenes are doing it".