Environment officials have issued a safety warning after a "devastating pollution incident" in a 20-mile stretch of river.

A leak of the chemical tributyltin has killed large numbers of fish in the River Blackwater between Coggeshall and Kelvedon in Essex.

Warning signs have been put up along the banks since a boy developed a skin rash after going fishing.

The Environment Agency says there is no risk to public water supplies, but has advised people not to use the river.

Recent spillage

The alarm was raised on Monday morning when a member of the public noticed dead fish.

The Environment Agency is now treating the pollution as a 'Category 1' incident - the top priority.

The agency is investigating whether the chemical, used as a wood preserver, could be residue from a recent spillage at a distribution depot.

Essex and Suffolk Water Company stopped taking supplies from the river before the pollution reached its plant.

The Environment Agency has warned farmers not to use the water for plants or livestock, and has advised swimmers and canoeists to keep out of the river.

Health problems

Dr David Irwin from Essex Strategic Health Authority told BBC Essex the chemical caused skin and eye irritation.

"If used in the open air, small droplets can cause irritation of the respiratory tract, but there have never been so far as we are aware any problems if it is eaten or drunk."

However, thousands of fish and some river animals have already died.

There is also a risk the pollution could spread into the Blackwater estuary and affect sea fish.

Prosecutions possible

Patrick Ripton, the environment protection manager for the area, said officials had decided against removing fish from the river as the pollution seemed to be lessening.

"This is a difficult exercise that would be stressful for the fish and could disrupt the balance of stock in the river."

He said the Environment Agency was now pumping in water from the Great Ouse river to speed up the flow.

It was also monitoring the situation in the Blackwater estuary.

Mr Ripton warned that prosecutions could follow.

He said: "Interviews are currently being carried out in connection with this devastating pollution incident."