The European Commission has warned Poland not to build a road through an environmentally protected area.
The Rospuda Valley is a haven for many animal species (Pic: P Malczewski)
EU Environment Commissioner Stavros Dimas said a by-pass near the north-eastern border with Lithuania would damage the natural habitat.
He has given the Polish authorities a week to stop the project or face action in the European Court of Justice.
Polish Prime Minister Jaroslaw Kaczynski had announced a referendum in the region on the project continuation.
"I urge the Polish Government to once more consider ways of building these bypasses without causing such serious environmental damage", Mr Dimas said.
"I believe that Poland has everything to gain by building new infrastructure without sacrificing its most precious natural heritage," he added.
Failure by Poland to respond satisfactorily could result in the Commission taking early action in the European Court of Justice.
Early action might include the Commission requesting the court to issue an order suspending the work on the projects pending a hearing of the case, according to the EU environment commissioner's office.
Polish authorities deny that the by-pass through the Rospuda Valley would damage the environment.
They say that the road will relieve congestion through the town of Augustow, which is crossed every day by some 4,500 heavy goods vehicles on their way to and from the Lithuanian border.
According to the Polish authorities, the 40 km (25 mile) bypass would cause minimal damage, as it would feature a bridge across the valley, rather than a ground-level highway.
But the British Royal Society for the Protection of Birds (RSPB) has criticised the project, saying it will endanger a number of animal species.
"The international transport corridor will put at risk strongholds of lynx, wolf and the most important European Union populations of two globally-threatened birds: the greater spotted eagle and aquatic warbler", the RSPB said in a statement issued last week.
The EU had already warned Poland in 2006 it could face legal action and penalties for failing to "adequately protect" its natural habitats.
EU commission biodiversity expert Agata Zdanowicz said Poland had so far failed to comply with the EU's Natura 2000 programme, describing the situation as "serious".
The Commission started an "infringement procedure" against Poland in April 2006.
This could lead to a court case and the blocking of EU funds for projects in Poland's environmentally fragile areas.
Natura 2000 is an EU-wide programme to safeguard the 27-nation bloc's most important wildlife areas and species.