French CRS anti-riot police officers patrol in the "Jungle" migrant camp in Calais | Philippe Huguen/AFP/Getty Calais warns UK over Brexit: ‘We will move border back’ French police complain of the burden of carrying out UK border checks on their side of the Channel.

PARIS — The president of France's northern Calais region warned Tuesday that if Britain voted to leave the European Union, France would no longer police the British border on the French side, as it currently does to keep illegal migrants out.

Under the terms of the "Le Touquet Treaty," British border checks take place on the French side of the Channel crossing, and vice versa.

However, French officials have long complained that the burden of policing the border is much heavier for France than for Britain because far more migrants are trying to enter British territory illegally than the other way around.

"If Britain leaves Europe, right away the border will leave Calais and go to Dover," Xavier Bertrand, the center-right president of the Nord-Pas-de-Calais Picardie region, which includes the city of Calais, told France Inter radio. "We will not continue to guard the border for Britain if it's no longer in the European Union."

Some 1,300 French police officers are under constant strain trying to stop migrants from crossing the border illegally to Britain. Fights among groups trying to board lorries, trains or boats bound for Britain are common, as are clashes with police.

Most of the migrants flock to Calais because they hope to be able to cross over to Britain, where many believe they will have better chances of finding work and winning political asylum than in France.

Migrant numbers have ballooned in the past two years, partly due to war in Syria. Poor living conditions in makeshift migrant camps in Calais and nearby Dunkerque, where more than 6,000 migrants are currently estimated to be living, have prompted the French state to install temporary living accommodations that can house about 2,000 in total.

On Tuesday Belgium announced it would temporarily re-establish border controls with France over concerns about the Calais camp, Agence France-Presse reported.

Bertrand's threat to move the border back to Britain reflects growing frustration among locals over migration, after street protests calling for the state to dismantle the camps and fix a long-term problem.

Moving the border back to the British side would free French police from the need to monitor the border, likely creating a bottleneck on the British side similar to the one that currently exists in Calais.

However, Bertrand has limited authority to act on the border question, even in the event that British voters did choose to exit the European Union in a referendum scheduled for June 23. The Le Touquet treaty was contracted between nations, not regions, and would likely require a higher authority to repeal.

Bertrand's office did not immediately answer a request to explain his authority on the border issue.