A diplomatic row erupted after a top ranking official said a historic border treaty would be ripped up and binned after Brexit.
Calais regional president Xavier Bertrand said France would renege on the terms of Le Touquet Treaty and refuse to police the frontier of a country outside the EU.
It would see British border checks pushed back across the Channel, raising the spectre of a migrant flood into the UK.
M Bertrand said: “If Britain leaves Europe, right away the border will leave Calais and go to Dover. We will not continue to guard the border for Britain if it’s no longer in the European Union.”
The treaty, signed in 2003, sees cross-Channel checks carried out at French ferry and train terminals. Scrapping it would cause a massive security nightmare with migrants free to cross the Channel from where they could launch attempts to sneak into the UK.
His comments comes after Prime Minister David Cameron claimed quitting the EU could see Calais-style migrant camps dotted along the Kent coast.
Mr Bertrand, a key ally of former Socialist French President Nicolas