British holidaymakers are being warned to stay away from beaches in northern France because of toxic fumes from rotting seaweed.
The problem has already led to one stretch of beach at Saint-Michel-en-Greve in Brittany being shut after a horse rider lost consciousness and his horse died.
Gilles Youenou, of the French Institute for Sea Research and Exploitation, said: "We would advise people to avoid the beaches in this area as the seaweed can give off hydrogen sulphide fumes.
"This is all a result of a problem with agriculture. Nitrates have got into the water and polluted it.
"A crust forms over stacks of rotting seaweed and when this crust is broken, fumes get out. It can be particularly lethal to dogs and other animals."
Jean-Francois Piquot, a spokesman for the French environmental group Eau et Rivieres, said that toxic seaweed has been present on beaches in Brittany for decades and is spreading.
He said: "There are about five beaches that are unusable. The problem is getting worse."
Up to 70,000 cubic metres of seaweed is cleared off about 70 beaches every summer in Brittany, according to Eau et Rivieres.
Pierre Philippe, of the Lannion hospital in Brittany, said hydrogen sulphide was as dangerous as cyanide.
He said he had treated several cases of poisoning caused by the seaweed among local residents, including a council worker paid to clear beaches of the algae who was taken to hospital in a coma.
Could this be a subtle rouse by the French to keep the British at bay??????