Opposition mounts against a proposal from the German and French interior ministers that called for the ability to reintroduce border controls in the Schengen area for up to a month at a time.

French President Nicolas Sarkozy has warned repeatedly in the run-up to Sunday's presidential election that if countries bordering non-Schengen nations fail to comply with their obligations to secure the frontiers appropriately, France have no choice but to protect its borders against illegal immigration by reinstituting passport controls.

This week, German Interior Minister Hans-Peter Friedrich is singing from the same hymn book. He and his French counterpart Claude Gueant wrote a letter to the Danish EU Council Presidency, calling for countries to have the right to reintroduce checks at the internal borders for no more than 30 days.

It is an election campaign maneuver, say critics, who reacted promptly. But Ska Keller, a German member of European Parliament for the Greens, said she believes there is more to the joint letter than just electoral politics.