Ther road would be a section of an international highway linking Warsaw to Helsinki via the Baltic states.

"The European Commission has asked the European Court of Justice to take an interim measure to ensure Poland does not go ahead with the construction of a road that will cause irreversible damage to the Rospuda valley," the European Union executive said in a statement.

Warsaw suspended the construction of a viaduct over Rospuda until Aug. 1 due to a bird nesting season in the region. It wrote to the Commission last week, refusing to promise that construction work would remain suspended.

Commission spokeswoman Barbara Hellferich told a daily news briefing it would be unprecedented for a member state to go ahead with such work in defiance with an EU order.

"This is uncharted territory. We have not had a case like that before," she said.

The Commission wants the work halted until the court delivers a final verdict on whether the motorway construction in the area is compatible with strict EU environmental laws.

Some analysts see the Rospoda dispute as a prelude to more conflicts between the EU and its new members from central and eastern Europe over the environment as they seeks to upgrade their often outmoded roads and railways.

The area is recognised by Brussels as having special environmental interest and has protected status under the EU's Natura 2000 network of conservation sites.

Warsaw insists it is breaking no EU rules. Poland's conservative government is under pressure from local residents who back the route as it would ease heavy lorry traffic that now cuts through towns.

Commission officials say privately that building the motorway through the valley could compromise EU funding for the whole Polish stretch of the planned Via Baltica motorway, which could amount to hundreds of millions of euros.