Poland must pay Brussels €500,000-a-day over its failure to close a coal mine on the Polish-Czech border, the European Court of Justice (ECJ) has ruled.
In March, Prague filed an injunction saying the Turow open-cast lignite mine drains groundwater away from surrounding areas.
Two months later, the ECJ told Warsaw to stop extracting brown coal.
On Monday, the court said Poland must pay the European Commission a daily fine of half a million euros after failing to cease activities.
The Turow mine is located in the southeast of Poland, close to the border with the Czech Republic and Germany.
It has been the subject of dispute between Warsaw and Prague, with the latter complaining it is damaging communities over the border.
In January, the German city of Zittau, just across the border from Turow, also took Poland to the same court on the same grounds. The Czech government filed the claim after talks with Warsaw were inconclusive.
Poland, which is reliant on coal for its energy needs, ignored the previous order from the ECJ to halt activities at the mine, and instead was hoping to reach an agreement with its neighbour.
State-run energy company PGE Group had contested the claim from the Czech Republic, arguing it would take years to phase out black coal in line with EU climate policies.
The country's climate ministry in April extended a mining concession for Turow until 2044.
Polish government spokesman Piotr Muller rejected the court's decision on Monday, saying that "suspending operations at the Turow mine would threaten the stability of the Polish energy system".
This would have "negative consequences for energy security for millions of Poles and for the whole EU", he added.
Planning to stick to coal mining beyond 2030 will mean the Polish region around Turow will miss out on a share of the EU's €17.5 billion "Just Transition" fund, which supports localities turning away from producing coal.
The ECJ has now ordered Poland to pay the daily penalty until it complies with the interim order of the court.
Poland generates around 70% of its electricity from coal, and was the only EU country in 2019 that did not commit to the goal of cutting the bloc’s net greenhouse gas emissions to zero by 2050.