Germany is backing the demand of French President Nicolas Sarkozy to give nations of the Schengen borderless zone more independent means to close their national territory "as a last resort."
Only days ahead of France's first round of presidential elections, during which illegal immigration is a major issue, Berlin and Paris sent a joint letter to the European Union demanding the "nonnegotiable" right to re-establish national border controls for a month if need be.
Sarkozy has said if he is re-elected, he could pull France out of Europe's border-free travel zone if more is not done to tackle illegal immigration. The purpose of the Schengen zone is to make travel easier among its 25 member countries.
In the letter by the French and German interior ministers, both nations said that any "threat to security and public order comes under national sovereignty," trumping any say by the EU or other multinational institution.
"It is nonnegotiable," said the letter to the EU presidency, which is now held by Denmark. It would exclude the Brussels-based European Commission in having any decisive say over rejecting the reintroduction of national controls at short notice.
Under the Schengen system, 25 EU countries opened their borders, making it possible for people to drive or fly from one country to another without taking out a passport or identity card. Normal requirements are only imposed on the outer borders of the entire zone of nations. But there have been increasing complaints that illegal immigrants have capitalized on this system by pouring into Europe across the Mediterranean Sea or into Greece from Turkey, which is not in the zone.
Germany's Interior Ministry spokesman, Hendrik Loerges, said that recent years have shown that an adjustment to the border rules is needed "because, at the moment, there are barely any possibilities to react if a Schengen member state doesn't adhere to the standards set out in the Schengen treaty."
Currently exemptions to the rules can be obtained for exceptional events like major summit meetings of government leaders and top sporting events such as the Olympic Games or European football championships.
On Friday, Spain announced it would restore border checks when Barcelona hosts the May 3 meeting of the European Central Bank, fearing it could attract protesters from abroad.
During the French election campaign, securing French borders has been a hot topic, especially for right-wing candidates, with government estimates that some 200,000 immigrants enter France each year.
Senior German opposition lawmaker Christine Lambrecht said that the Franco-German "populist proposal ... is nothing more than propaganda for Sarkozy in the French presidential election campaign."
"The German and French interior ministers should drop this right-wing populist discussion and give up the plan quickly," said Lambrecht, of the center-left Social Democrats. "Possible refugee problems cannot be solved this way."
Schengen has become a focal point for populist politicians and some governments across the continent to denounce cross-border crime.
The Netherlands has led opposition to Romania and Bulgaria joining the borderless free-travel zone, arguing that would open the 25-nation zone to an increase in organized crime and corruption. It has been backed by Finland.
Daniel Woolls from Madrid and Geir Moulson in Berlin contributed to this story