The EU's top court on Monday warned Poland's right-wing government to "immediately" stop logging in one of Europe's last primeval forests or face fines of up to 100,000 euros ($118,000).
The case is the latest in a string of issues causing tensions between Warsaw and Brussels, which has watched the Polish administration's recent judicial reforms with alarm.
"Poland must immediately cease its active forest management operations in the Bialowieza Forest, except in exceptional cases where they are strictly necessary to ensure public safety," the European Court of Justice said.
"If there is found to be an infringement, the court will order Poland to pay to the (European) Commission a penalty payment of at least 100,000 euros a day," the Luxembourg-based court added.
The court first ordered Warsaw to suspend logging in the forest on July 27 pending a final judgement.
The EU had taken Poland to court arguing that the operations were destroying a forest that boasts unique plant and animal life, including the continent's largest mammal, the European bison.
The European Commission, the executive arm of the 28-nation European Union, has warned Poland to comply or see the logging issue added to a broader EU case against Warsaw over the rule of law.
The European Parliament last week voted to start an EU sanctions procedure over Warsaw's controversial judicial reforms that could eventually suspend Warsaw's voting rights in the bloc.
Adding to the trouble, EU President Donald Tusk, a former liberal prime minister of Poland, on Sunday questioned whether tensions between Poland's government with Ukraine and the EU were part of a "Kremlin plan".
Polish PM Beata Szydlo said that by "using his position to attack the Polish government, he's attacking Poland."