Tests were ordered on the foul-smelling algae, which green groups blame on nitrates fertilisers used by local farmers, after a horse apparently died from inhaling fumes on a beach in Saint Michel de Greve in Brittany.

Results showed the seaweed in Saint Michel was giving off dangerous levels of hydrogen sulphide (H2S), sometimes referred to as "sewer gas" because it is produced by the breakdown of putrified waste material.

"Measurements carried out on site ... showed in several places that the gas released by sediment containing the decomposing algae could be dangerous," said France's national institute for environmental threats, INERIS.

Several points on the beach tested positive for hydrogen sulphide at a concentration of 1,000 parts per million, a level that "can be deadly in a few minutes," the report said.

INERIS recommended the area be cordoned off as a short-term precautionary measure, and for workers charged with clearing the algae be equipped with hydrogen sulphide detectors.

Prime Minister Francois Fillon and the ministers for health, agriculture and the environment visited the site in Saint Michel on Thursday to evalute the situation and decide what safety measures to adopt.

The build-up of rotting weed on shores in more than 80 towns around Brittany has worried residents and threatened the region's lucrative tourist industry, with part of the coastline already declared off-limits.

Green groups blame nitrate pollution caused by intensive agriculture -- especially among pig farmers -- and have accused the government of turning a blind eye to an "environmental cancer."

The government was spurred to act after a horse and rider fell onto a patch of the algae on July 28.

The horse died immediately, while 28-year-old horseman Vincent Petit lost consciousness and was pulled to safety by nearby workers.

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