Guards could return to Europe's border-free area as EU authorities unveile proposals tomorrow to ease French and Italian fears about a potential deluge of penniless north African migrants.

A symbol of European unity, the passport-free travel region has faced major strains with the arrival of thousands of Tunisians in Italy since a revolution toppled Tunisia's dictator in January.

With Italy's neighbours fearing migrants will take advantage of Europe's open borders, the European Union's executive Commission will propose a mechanism giving states more flexibility to temporarily restore controls.

After a spat over Italy's decision to issue residency permits to thousands of Tunisians hoping to reach France, French President Nicolas Sarkozy and Italian Prime Minister Silvio Berlusconi sent a letter to the commission last week calling for reform of the Schengen borderless area.

European Commission president Jose Manuel Barroso responded in a missive to the two leaders that the temporary restoration of borders would be "a possibility among others" to reinforce Schengen.

Germany, however, has warned against undermining Europe's liberalised travel policies.

"If you can improve the Schengen system then that is good and you should," Foreign Minister Guido Westerwelle said last week. "But travel freedom in Europe is such an important achievement that it should not be up for renegotiation."

Members of the European Parliament also voiced concerns.

"Schengen should not be weakened," said civil liberties committee member Carlos Coelho. "Schengen is free movement and, like the euro, is one of the symbols of Europe."

First signed in 1985 as a giant step towards European integration, the Schengen treaty has opened passport-free travel to 400 million people in 22 EU nations plus Iceland, Norway and Switzerland.

Currently, only a "serious threat" to public order or internal security can be used as exceptional justification for temporary border controls -- allowing authorities to restore passport checks to stop hooligans from attending football matches or protesters from disrupting summits.

France, arguing that the safeguard clause is too restrictive, says states should be allowed to restore border checks when another nation fails to police its section of Schengen's external border.

While the influx of Tunisians in Italy's tiny of Lampedusa has been the focus of the debate, Schengen was already under pressure with Greece struggling with migrants crossing its porous border with Turkey.

Fears that Europe's open internal borders may also facilitate criminal activities have seen Paris and Berlin refuse to allow Romania and Bulgaria to join the Schengen area until they clean up corruption.

In a draft policy paper obtained by AFP, the commission calls for "effective and credible" external borders and says cooperation between national authorites must improve while a European system of border guards should be considered.

The capacities of the pan-European Frontex border agency should also be reinforced, the EU executive says.

But a control mechanism must be put in place when a member state "is not fulfilling its obligations to control" the external border, or when the border comes under "unexpected and heavy pressure due to external events."

"As a last resort in truly critical situations," a system for a "coordinated and temporary" reintroduction of border controls may be needed, the document says.

The proposals will be discussed at a special meeting of EU interior ministers on May 12 in Brussels before a summit of European leaders in June.

"The goal is to move as fast as possible to reinforce Schengen governance," a European diplomat said.