"Calling into question the Touquet deal on the pretext that Britain has voted for Brexit and will have to start negotiations to leave the union doesn't make sense," the Socialist president said after a summit in Brussels.
The Touquet accord, signed in 2003 between France and Britain, effectively moved Britain's border to the northern shores of France, where migrants fleeing war and poverty in the Middle East, Africa and Asia have massed in their thousands.
It notably allows for British border controls in French port cities.
Undoing it would return British police, customs officials and sniffer dogs to their home across the English Channel - and open the door to the thousands of migrants camped in Calais and other areas along the French coast who are all hoping to make it to the UK.
French Interior Minister Bernard Cazeneuve confirmed there would be no changes to the accord as he met with Calais officials in Paris on Wednesday.
"The border at Calais is closed and will remain so," he said.
Political price for French
The French government is paying a substantial political price for allowing the border to remain in France, with the far-right exploiting the issue in port regions and local politicians venting their fury.
Calls by some French officials to challenge the treaty in the wake of Britain's seismic vote to leave the bloc would only encourage human traffickers, Cazeneuve said, potentially triggering a "fresh influx of migrants into Calais" and a spike in potentially life-threatening attempts to cross the Channel.
Many in northern France believe that getting rid of British border controls in France would trigger an exodus of migrants from the squalid "Jungle" camp in Calais and other makeshift camps along the coast, easing the financial and security burden on France.
French authorities began dismantling the "Jungle" in March, but several thousand migrants refused to move out and remain holed up there, hoping to find a way to Britain.
Calais Mayor Natacha Bouchart told reporters after the meeting with Cazeneuve that something had to change.
"We can't take it anymore... I demand that we renegotiate the Touquet accord," she said.
(FRANCE 24 with AFP)