A horse died on Saint-Michel-en-Greve beach late last month
French Prime Minister Francois Fillon has announced his government will pay for cleaning French beaches polluted by a toxic seaweed.
He was speaking during a visit to a beach in Brittany where the green algae has been proliferating for years.
A new study says the algae can pose a potentially fatal health threat.
Local communities in Brittany have long criticised the government for failing to address the problem, calling on it to help clean up the beaches.
The killer algae is also affecting the English coastline - particularly Dorset, Hampshire, the Isle of Wight and West Sussex.
On Wednesday, the UK Environment Agency said the algae was threatening wildlife along the coast.
In France, Mr Fillon visited a beach in Saint-Michel-en-Greve to see the situation for himself.
"The state will assume all of its responsibilities and will take charge of the clean-up of the worst affected beaches, where there could be a public health risk," he said.
On the same beach, a horse-rider was rendered unconscious and his mount died after slipping on the algae late last month, apparently after inhaling toxic gas released by the rotting seaweed.
The incident prompted the French government to commission a study on the algae's toxicity, which stems from a noxious gas - hydrogen sulphide - being emitted as the seaweed decomposes.
Researchers from France's National Institute for Environmental Technology and Hazards (Ineris) found a potentially lethal concentration of the hydrogen sulphide on parts of the beach.
They studied algae samples from the Saint-Michel-en-Greve beach, and found a concentration of the gas of up to 1,000 parts per million in some areas of the beach.
If inhaled, such a concentration of gas "can be deadly in few minutes", said their report, which was published on Thursday.
The report recommended banning access to potentially dangerous areas, and equipping algae clearing workers with gas detectors.
Intensive agriculture is often blamed for the proliferation of algae along France's coasts.
The seaweed thrives on high levels of nitrates used in fertilisers and excreted by the region's high concentration of livestock.
The UK Environment Agency said tighter controls on farming fertiliser and sewage plants should begin to starve the algae of the nutrients it needs to survive.