The French authorities are clamping down on illegal “magnet fishing” because of the danger that enthusiasts, who use large magnets to remove debris from rivers and canals, may pull up unexploded bombs or shells.

Police had to clear a large area in the western city of Nantes after a man hauled a Second World War mortar bomb out of the River Erdre on Saturday.

Specialist deminers were called in to dispose of the unexploded 10-inch-long finned projectile.

“Magnet fishing” is an increasingly popular pastime but its dangers were highlighted when a 29-year-old man was blinded and severely burned on the face after he brought up a First World War shell containing toxic mustard gas in northern France in May.

Hobbyists tend to be treasure hunters or environmentalists, but the practice is illegal in France without a permit.

In England and Wales, it is banned in many rivers and canals under the control of the Canal & River Trust, and is punishable by a £25 fine.