Poland has ordered the Russian energy giant Gazprom to pay a €6.5bn fine over the construction of the Nord Stream 2 gas pipeline.
Poland's competition watchdog says the project will unfairly increase the firm's dominance of the Polish market, with potentially serious consequences for the country's economy.
Gazprom is leading the Baltic Sea pipeline project, which would allow Germany to double its gas imports from Russia.
The company says it will appeal the decision. It insists there are no grounds for the fine and says the penalty is a politically-motivated move to please the US government.
The German government meanwhile has so far remained very cautious about questioning the project, even though its relations with Moscow are currently at a low ebb.
One of the causes of current tension is the murder of a Chechen veteran Zelimkhan "Tornike" Khangoshvili in a Berlin park by an alleged operative of Russia's GRU federal agency, whose trial began in the German capital on Wednesday.
The poisoning of Russian opposition leader Alexei Navalny - who was treated in a Berlin hospital - has also strained relations. Heiko Maas, the German foreign minister, was quoted by Bild in September as saying he hopped Russia "do not force us to change our position on Nord Stream".