Warning notices have been posted at Coalhouse Fort, East Tilbury, following the discovery of toxic blue-green algae in the moat.
Members of the public alerted Thurrock Council after seeing dying fish floating on the surface of the water.
Officers of the Environment Agency were called in who confirmed the presence of blue-green algae bloom in its early stages.
A council spokeswoman said: "We are putting up signs warning people not to go into the moat themselves or allow their pets to swim there."
Published Wednesday August 14, 2002
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