A stretch of beach had to be closed after a horse rider lost consciousness as a result of the putrefying algae. Sadly his horse was killed. Local residents have also been treated in hospital.

The dangers were highlighted after Vincent Petit, 27, a veterinary surgeon from Paris, said that rotting seaweed a metre deep had killed his horse last week as he rode across St-Michel-en-Grève beach, Brittany. Mr Petit lost consciousness and was pulled off the beach. A post-mortem on the horse showed that it had died of pulmonary oedema caused by inhaling hydrogen sulphide given off by the rotting seaweed.

This very toxic gas, which smells like rotten eggs, attacks the respiratory system and can kill a man or an animal in minutes. Hydrogen sulphide is said to be as dangerous as cyanide. Some scientists believe that a build-up of hydrogen sulphide in the atmosphere wiped out the dinosaurs 300 million years ago.

The green seaweed is spreading across the region’s beaches as nitrates pollute the water supply as a result of intensive agriculture.