Poland will hold a referendum in its north-eastern region on whether to build a controversial road through an environmentally protected area.
Rospuda Valley is a haven for many animal species (Pic: P Malczewski)
Prime Minister Jaroslaw Kaczynski said the dispute between the government and environmentalists had become a "national problem".
The European Union has warned Poland not to build the road through the Rospuda Valley.
The area of almost untouched peat bogs and woods borders on Lithuania.
"According to law, this controversy can be solved in one way only: by asking the opinion of the residents of the region", Prime Minister Kaczynski told a news conference on Friday.
The project to build a trans-European highway through the area predates Poland's accession to the European Union in 2004.
The EU Environment Commissioner, Stavros Dimas, said the project would mean a "major catastrophe in this precious area of Poland".
Polish authorities say they want to 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.
They say the 40-kilometre (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 on Thursday.
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.