The European Commission is seeking a court order to prevent Poland re-starting work on a road through a protected wildlife area.
Lynx, wolves and rare warblers live in the valley (Picture: P Malczewski)
The road - which carries traffic from Warsaw to Helsinki - goes through the Rospuda Valley - a peat-bog area that is home to rare plants and animals.
Warsaw halted work for the bird-nesting season but plans to resume on 1 August.
It says environmental damage would be minimal, because the plan is for a viaduct not a road at ground level.
"The EU is asking the European Court of Justice to issue an injunction to prevent irreversible damage to a unique environmental site," said Barbara Helfferich, spokeswoman for Environment Commissioner Stavros Dimas.
Funding at risk
The Commission wants work suspended until the European Court decides whether the road - a 40km (25-mile) bypass around the town of Augustow - complies with strict EU environmental laws.
The town is crossed every day by some 4,500 heavy goods vehicles on their way to and from the Lithuanian border.
The British Royal Society for the Protection of Birds (RSPB) has complained the project puts at risk strongholds of lynx and wolf and "the most important European Union populations of two globally-threatened birds: the greater spotted eagle and aquatic warbler".
Commission officials say Poland could lose hundreds of millions of euros of funding for other stretches of the planned Via Baltica, from Warsaw to Helsinki, if it goes ahead with the Rospuda project.
The Rospuda section is being built without EU funds, but Poland hopes Brussels will co-finance the construction of other parts.