The harvesting of scallops, oysters and mussels at a Dorset harbour has been stopped after high toxin levels were found in mussels.
Portland Harbour is closed to the gathering of filter feeding molluscs, the port health authority said.
Routine testing of molluscs off the Dorset coast has indicated the presence of Amnesic Shellfish Poisoning (ASP) toxin in the algae.
It can be a risk to public health if consumed in "sufficient quantity".
Dizziness and confusion
The presence of ASP toxin follows the growth of an "algal bloom" in the English Channel, Weymouth Port Health Authority said.
It said it was not possible to define the exact extent of the bloom and has warned commercial harvesters to ensure each batch is tested for the toxin, or face committing an offence under food safety and hygiene regulations.
ASP can cause neurological symptoms, such as dizziness and confusion, and in "very rare cases" death, the authority said.
The harbour closure does not include The Fleet oyster beds where ASP toxin has not been detected, it added.
It said it was working closely with the Food Standards Agency, the Inshore Fisheries and Conservation Authority and the Marine Management Organisation to monitor the situation.