Fishermen on the Essex side have been ordered not to collect cockles, mussels, clams and oysters because of the risk of severe diarrhoea and sickness in anyone who eats them.

Holidaymakers and visitors to places such as Southend are being warned not to collect shellfish from the beaches.

The animals have been found infected with diarrhetic shellfish poisoning (DSP) which may be linked to climate change.

Cockle season

The contamination was discovered after samples were collected by scientists from the London Port Health Authority (LPHA).

It coincides with the start of the cockle-fishing season, but does not mean the boats are without work.

They can still fish on the Kent side of the estuary which is so far unaffected.

There have not been any recorded cases of people becoming ill from eating the shellfish this year.

It is the sort of thing that can be serious for those who are vulnerable such as old people and children.

Spokesman, London Port Health Authority

A spokesman for the LPHA told BBC News Online: "This is a preventative measure.

"The symptoms are very similar to a violent bout of food poisoning.

"It is the sort of thing that can be serious for those who are vulnerable such as old people and children.

"It is something for which we regularly test and regularly monitor shellfish."

Diners made ill

The toxins, which are produced by species of algae, accumulate in shellfish and are not destroyed by cooking.

Cockles could be off the menu for weeks

Three years ago, 49 diners at two London restaurants suffered acute nausea, vomiting, diarrhoea, abdominal pain and feverishness lasting eight hours.

All had been served dishes of mussels originating from the UK.

DSP toxins were first detected in UK shellfish in a batch of cockles from the Thames Estuary in June 1991 and the problems appears to be getting worse.

Fishing industry hit

There were similar difficulties in the estuary last year although the fishing ban was eventually lifted.

The contamination is a serious blow to the fishing industry.

Some 7,000 tonnes of cockles were collected from the Thames Estuary last year and the main beds are on the affected side.

Further tests are now being carried out to see whether the contamination has spread.

Officials from the LPHA say it may be some weeks before the problem disappears.