The Environment Agency says it is worried by the spread of algae that is threatening to choke wildlife along the south coast of England.
The algae is thriving on nutrients released by farmland fertilisers and sewerage systems.
Hundreds of acres of mudflats in Dorset, Hampshire, the Isle of Wight and West Sussex have been taken over.
The agency hopes new controls on farming and sewage will help to starve the algae.
Large areas of the French coastline in Brittany have also been been badly affected.
Walkers in the UK and France say the green slime is ruining beautiful coastlines.
The Environment Agency says there is no danger to people or their pets - but has warned walkers to avoid the areas if they smell rotting eggs as a precaution.
The algae has been building up in some areas since the 1960s. Tonnes of the seaweed are now threatening important habitats for plants and wildlife.
The Environment Agency's Dave Lowthion is helping the assess how far the algae has spread.
'Decade to recover'
He said: "When you get a deep layer of the seaweed it actually stops the oxygen getting into the mudflats.
"And because of that the worms, the shrimps etc that live there, some of the sensitive species die."
Birds which feed on mudflats are finding the thick layers of algae hard to break through, so they cannot get to the surviving creatures.
The algae is growing where tides are not strong enough to break up beds of the seaweed.
The areas affected are Chichester and Pagham harbours in West Sussex, Portsmouth and Langstone harbours and the Hamble estuary in Hampshire, Newtown and Bembridge harbours and the Medina estuary on the on the Isle of Wight and Poole Harbour and Holes Bay in Dorset.
It is hoped that tighter controls on farming fertiliser and sewage plants should begin to starve the algae of the nutrients it needs to survive.
But experts think it will be a decade before some mudflats return to normal.
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