A committee of the European Parliament has called on Poland to find a different route for a planned motorway across the north of the country.
Lynx, wolves and rare warblers live in the Rospuda valley
The parliament's petitions committee said an alternative route - under consideration in the 1990s, but later rejected - was the "only suitable" one.
Petitions to the parliament said the route chosen by the Polish government would damage key wildlife habitats.
The committee agreed, and called for the "unique" sites to be preserved.
It said "no convincing argument" was given for a decision to route the Via Baltica transport corridor, linking Warsaw to the Baltic states, through the towns of Bialystok and Augustow.
"There appears to be a systemic problem which has lasted for a number of years... which has... blocked any sensible consideration of alternative options," a report approved by the committee said.
The report said an alternative route - passing close to the towns of Lomza, Chodorki and Suwalki - was the "only suitable option if one is to avoid irreversible damage to the unique and valuable sites protected under EU environmental directives".
It named in particular the Rospuda valley wetlands, near Augustow, and the Knyszynska forest, near Bialystok, home to rare European bison.
Campaigners welcomed the report, saying the alternative route was shorter and potentially cheaper, and "would not cut... wildlife refuges in half".
The Polish government halted work in July on a bypass around the town of Augustow through the Rospuda valley, after the European Commission asked the European Court of Justice to intervene.
However, a petitioner who attended the committee meeting in Brussels, Malgorzata Znaniecka, said work on other parts of the motorway was still continuing.