Forming part of an international motorway linking Warsaw to Helsinki via the Baltic states, the ring-road scheme would include a 500-metre flyover across the valley where numerous species of migratory birds nest.

"We realise a ring-road is needed and have been proposing an alternative route less damaging to the environment than that through Rospuda Valley," environmentalist Jan Jakiel told Reuters.

Dozens of environmentalists have pitched tents and threatened to chain themselves to trees if road builders move in with chainsaws and bulldozers, said news channel TVN24, which also acknowledged many local residents favoured the plan.

European Commission President Jose Manuel Barroso recently told Polish authorities the EC regarded the project as an infringement of Poland's Natura 2000 programme, designed to protect nature reserves such as Rospuda.