The Kremlin has reacted furiously after a Polish government body advised using a different name for Russia's exclave of Kaliningrad on the Baltic Sea coast.
The Polish committee said the city and wider area should instead be called Królewiec.
This was the area's traditional name, it said, and the decision no longer to use an "imposed name" was partly a result of Russia invading Ukraine.
Russia said the decision was "bordering on madness" and "a hostile act".
"We know that throughout history, Poland has slipped from time to time into this madness of hatred towards Russians," said Kremlin spokesman Dmitry Peskov.
For hundreds of years before World War Two, the area was known as Königsberg and was part of East Prussia. Królewiec is the Polish translation of Königsberg.
However, after World War Two, the city and wider region were placed under Soviet administration. The Soviets renamed it Kaliningrad after Mikhail Kalinin, one of the leaders of the Bolshevik revolution.