The "winds of populism" are threatening the EU's Schengen Treaty on open borders, European Council President Herman Van Rompuy has warned.
He was speaking in Romania, which is lobbying with neighbouring Bulgaria to join Schengen this year.
Mr Van Rompuy, who chairs EU summits, said that in Europe's "space of freedom and rule of law... there is no room for stigmatisation of foreigners".
Anti-immigration parties have polled strongly in France and the Netherlands.
In the first round of the French presidential election on Sunday the far-right National Front leader, Marine Le Pen, took nearly 18% of the vote.
That result prompted both Germany's Chancellor Angela Merkel and EU Commission President Jose Manuel Barroso to warn of the threat of "populism".
Mr Van Rompuy said "regrettably the winds of populism are affecting a key achievement of European integration: the free movement of persons within our borders".
He was speaking on the eve of EU justice ministers' talks in Luxembourg, expected to cover Schengen, which is now under review.
According to an EU statement, the ministers are due to approve "a roadmap to ensure a coherent EU response to continued migratory pressures".
France campaign issue
The Schengen rules apply to 25 countries, including non-EU states Iceland, Norway and Switzerland. The UK and Republic of Ireland do not participate.
Schengen scrapped routine passport checks at national borders and only a threat to public order is seen as grounds for restoring border controls.
But the economic crisis has increased pressure to keep illegal migrants out. Last year France and Italy led the calls for more surveillance of Europe's borders.
Germany and France want the EU to allow Schengen states to reimpose border controls for up to 30 days to prevent any major influx of illegal immigrants.
On Monday, French President Nicolas Sarkozy called for tougher border controls, a day after losing the first round of the election to Socialist leader Francois Hollande.
"The French don't want a Europe that leaks like a sieve anymore. That's the message I've heard," he said at an election rally.
"If Europe can't control its borders then France will do so. A Europe that doesn't control its migration flows is finished."
Last year the Netherlands government blocked an EU deal to let Bulgaria and Romania join Schengen, arguing that they needed to do more to prevent illegal migrants entering and tackle organised crime, including people trafficking.
An EU decision on the matter is expected in September. Fresh doubts have arisen because the Dutch coalition government has collapsed and the anti-immigration Freedom Party of Geert Wilders is one of the Netherlands' major parties.
Mr Van Rompuy backed the European Parliament's call to let Bulgaria and Romania join Schengen this year. "All European leaders have underlined more than once in our meetings that all legal conditions for accession have been met," he said.