Slovakia's government has barred the Hungarian president from making a controversial visit, BBC reported.
President Laszlo Solyom had planned to visit a part of Slovakia with a large ethnic Hungarian population, to unveil a statue of the first Hungarian king.
Prime Minister Robert Fico said the plans were a provocation.
Bilateral relations have been strained over Slovakia's treatment of its ethnic Hungarians, who make up about 10% of the population.
"The foreign ministry today handed a diplomatic note to the Hungarian ambassador saying Slovakia will not let the Hungarian President Laszlo Solyom enter the country today," Mr Fico was quoted as telling reporters by AFP news agency.
"If he decides to ignore the note it would be a violation of international law and Slovakia's sovereignty."
Slovakia's Interior Minister Robert Kalinak added: "If he enters Slovakia's territory he will be warned that his presence is not welcome and will be asked to leave."
Mr Solyom had been due to attend the unveiling of a statue of the mediaeval monarch Saint Stephen in the southern town of Komarno, on the River Danube which separates the two countries.
Slovakia blocks Hungarian visit
