A floating barrage of buoys was stretched across a river near Etaples and Le Touquet in northern France.
There are fears smugglers are using waterways to launch boats after a police crackdown on the beaches.
But just yards away, on the Channel side of the blockade, is a runway into the water and a car park.
The site is significant because many smugglers will drop dozens of migrants off by vehicle moments before they attempt to launch a dinghy destined for the UK.
Alp Mehmet, chairman of Migration Watch, told the Daily Express: "Is this an indication of how the French regard illegal migration? It's time they took it seriously and put the millions we give them to better use.
"They could start by stopping more boats from setting off illegally from France. They should also accept back those who make it into British waters. They might then find fewer enter France with the intention of making their way to the UK."
The discovery heaps pressure on French authorities as it emerged a record number of migrants have crossed the Channel at the start of this year. So far 11 people have died trying to reach our shores in 2024, according to migrant charity Care4Calais -including a seven year-old girl killed last week when a flimsy inflatable collapsed in the River Aa.
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