Poland's rightwing government broke the law by logging in one of Europe's last primeval forests, the legal advisor to the EU's top court said Tuesday, setting up a new clash between Brussels and Warsaw.

Logging in the Bialowieza Forest 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.

Bialowieza, which straddles the border with Belarus, includes one of the largest surviving parts of the primeval forest that covered the European plain 10,000 years ago.

The decisions by the Polish government to allow logging in the forest "infringe EU law", said Yves Bot, advocate general to the European Court of Justice.

Bot "proposes that the court should rule that Poland has failed to fulfil its obligations" under EU environment law, the court said in a statement.

It added that the logging was "liable to result in a deterioration of the breeding sites of protected species" in the vast woodland, which is home to 800 European bison, the continent's largest mammal.

The Luxembourg-based EU court often, but not always, follows the legal opinions of the advocate general when making its final decision. Its ruling is expected at a later date.

Bialowieza, which straddles the border of Poland and Belarus, includes one of the largest surviving parts of the primeval forest that covered the European plain 10,000 years ago WOJTEK RADWANSKI, AFP/File

The Polish government says 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.

- 'Exploitation of the forest' -

Poland said it would respect the eventual judgment, but insisted it was only felling trees for public safety reasons in accordance with the EU injunction.

"We will analyse the advocate general's opinion closely but what I can say now is that Poland will respect the definitive judgment of the court on the Bialowieza forest," new Environment Minister Henryk Kowalczyk said in a statement.

"All previous actions were taken to keep the forest in the best possible state for current and future generations."

Activists, scientists and other critics allege Poland is engaged in commercial logging.

“We are not surprised by this important legal opinion," Agata Szafraniuk, a lawyer for the ClientEarth group, told AFP.

A coalition of environmental groups including Greenpeace Poland, WWF Poland, ClientEarth and Greenmind urged Kowalczyk to "immediately cancel decisions allowing the exploitation of the forest and to start steps to enlarge the national park area."

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 100,000 euros ($118,000) a day if it continued.

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.