The incident happened on Wednesday when a lifeguard spotted a suspicious-looking shape in the water.  

“While looking out at the sea I saw a round fin,” Franck Peslherbe told local newspaper La Voix du Nord. “I said to myself: that’s not a seal.”

He contacted his colleagues and asked them to give a second opinion. They reached the same conclusion and the evacuation order was given.

The scary visitor was almost certainly a basking shark, according to experts, a species which can grow to up to 12 metres in length, making it the second largest living fish. The shark spotted in the Channel was around two to three metres, according to witnesses, so probably a younger fish.

“It’s completely harmless,” said Dominique Mallevoix from Nausicaa, the sea-life and marine centre in nearby Boulogne-sur-Mer. “The basking shark only eats plankton.”

After two hours, the shark disappeared from view and the all-clear was given for swimmers to go back into the water. At around 5pm, a kitesurfer emerged from the water in a state of panic. He had reported coming face-to-face with the shark, which led to a second evacuation of the water.

Basking sharks are rare in the English Channel, although not unheard of. “It’s likely that they were following the movements of plankton so they could eat,” marine scientist Agathe Lefranc told the newspaper.