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The naturally occurring toxins, found in a sample of the molluscs, can cause Diarrheic Shellfish Poisoning (DSP).

Anyone experiencing DSP symptoms which include diarrhoea and vomiting, should consult their GP, Weymouth & Portland Borough Council said.

A temporary ban placed on harvesting mussels, clams, scallops and oysters from The Fleet does not include crabs and lobsters or gastropods like whelks.

A council spokeswoman said: "Monitoring of the situation continues and the closure notice will be lifted as soon as it is safe to do so.

"The DSP symptoms usually start to take effect in the body between 30 minutes and several hours after ingestion of affected shellfish."

Warning notices have also been posted "at strategic locations" warning members of the public of the dangers.

Anyone who needs more information should contact Weymouth Port Health Authority, the spokeswoman added.