The 13ft barrier, dubbed The Great Wall of Calais, was built using £2.3million of UK taxpayers’ money.
It runs for well over half a mile beside the N216 motorway, which ends at the town’s ferry port terminal.
But a probe has found that steel doors set into it as access points for French officials are a glaring weak point.
On the side of the wall facing away from the road the doors look like featureless sheets of plain metal.
But on the traffic side they have handles with simple catches which can be opened without a key.
Migrants have discovered that all they have to do is gather at a door and send one of their mates running to the end of the wall.
He skirts round it, hares back to the door and opens it to let them all pour through to the road where they can jump on trucks – the very thing the wall was designed to prevent.
The embarrassing security lapse is said to be an open secret among Calais’ community of migrants desperately trying to get to Britain illegally.