At least 10 people were injured when the fire tore through closely-packed huts at the Grande-Synthe camp, near the port of Dunkirk.
Last month officials said the camp would be dismantled because of unrest.
The French north coast has been a magnet for migrants trying to reach Britain.
"There is nothing left but a heap of ashes," said Michel Lalande, prefect of France's Nord region.
"It will be impossible to put the huts back where they were before," he added.
