The European Commission has taken Poland to Europe's top court in an effort to head off the construction of a highway through an environmentally sensitive area.
EU Environment Commissioner Stavros Dimas said the legal action is regrettable, but necessary to protect "precious natural sites of European significance."
The highway is to link Warsaw with Helsinki through the Baltic countries. Polish authorities plan a bypass through the Rospuda Valley to relieve traffic in the city of Augustow in northeastern Poland.
The Rospuda Valley is an area of peat bogs and forests and the home to rare plants and wildlife. Both Poland and EU authorities have declared it an important conservation site.
Poland's top environment official has said he is convinced the bypass is within the law.
Polish Prime Minister Jaroslaw Kaczynski has called for a local referendum on the issue.
Some information for this report was provided by AFP and AP.