The European Court of Justice ruling said that by increasing logging in the Bialowieza Forest, Poland failed to fulfil its obligations under EU directives to protect natural sites of special importance.
Poland's right-wing government broke the law by logging in one of Europe's last primaeval forests, the EU's top court ruled on Tuesday, setting up a fresh clash between Warsaw and Brussels.
Logging in the Bialowieza Forest, a UNESCO world heritage site, began in May 2016 but the European Commission took Poland to court last year arguing that it was destroying a forest that boasts unique plant and animal life.
"The forest management operations concerning the Puszcza Bialowieska Natura 2000 site that have been undertaken by Poland infringe EU law," the European Court of Justice (ECJ) said in a statement.
"The implementation of those operations in fact results in the loss of a part of that site," the Luxembourg court added.
In Warsaw, the Polish government said it would comply with the court ruling, which it must do "without delay" in order to avoid financial penalties.
Bialowieza, which straddles the border with Belarus, includes one of the largest surviving parts of the primaeval forest that covered the European plain 10,000 years ago.
Its vast woodland is home to 800 European bison, the continent's largest mammal.
The government of the Law and Justice party (PiS) has said it is clearing dead trees to contain damage caused by a spruce bark beetle infestation, as well as to fight the risk of forest fires and preserve road traffic.
Activists, scientists and other critics allege Poland is engaged in commercial logging.
The court formally ordered Poland in July to suspend logging pending a final ruling, and in December it threatened the government with fines of up to $118,000 (100,000 euros) a day if it continued.
The EU judges found that the Polish government failed to carry out "an appropriate assessment" of the impact of its forest management plan before launching it in 2016.
It said Warsaw's argument for felling trees failed to show the management plan was justified by the spread of the spruce bark beetle.
Poland 'will comply'
The court also found that the Polish plan failed to comply with EU obligations protecting nests, eggs and breeding places of bird species living in the forest.
Poland said it would "comply" with the ruling.
"We have just received the official draft of the decision, which will be carefully analysed at the ministry," the environment ministry's spokesman Aleksander Brzozka said.
"But, as (Environment) Minister Henryk Kowalczyk has said several times, Poland will comply with the ECJ decision," Brozka added.
Andreas Baumueller, an executive at the European office of conservationist group WWF, said: "Today is a clear victory for Europe's wildlife. Nature cannot be ignored and neither can EU law."
The case is the latest in a string of issues causing tension between Warsaw and Brussels, which has watched the Polish administration's recent judicial reforms with alarm.
In December, after months of warnings, the European Commission launched an unprecedented procedure against Poland that could strip Warsaw of its voting rights in the bloc if it does not scrap the reforms.
EU ministers are considering the rule-of-law issue in Luxembourg on Tuesday.
Poland's right-wing government has also faced an international row over a law making it illegal to attribute Nazi crimes to the Polish state.
Mateussz Morawiecki, who has eased tensions on the court reforms since becoming premier in December, fanned the flames by saying there were "Jewish perpetrators" as well as Polish ones in the Holocaust.