The Food Standards Agency is advising that pregnant women, women who intend to become pregnant, infants and children under 16 to avoid the fish.

Officials said the advice was precautionary and follows a survey, which found high levels of mercury in those fish.

In a statement, it said: "Large predatory fish like shark, swordfish and marlin can contain relatively high levels of mercury in the form of methylmercury, which can harm the nervous system of an unborn child

"Infants and children may also be at greater risk from mercury poisoning because they eat more food relative to their body size in comparison with adults."

Officials said occasional consumption of shark, swordfish or marlin as part of a balanced diet by any other adults is unlikely to result in harmful effects.

But they advised people against eating more than one portion each week of either shark or swordfish or marlin.

The FSA surveyed 336 fresh, frozen and processed sea fish and shellfish for mercury content, including trout, salmon, tuna, halibut, hoki, sea bass, lobster, mussels and prawns.

Levels of mercury in fish other than shark, swordfish and marlin did not give cause for concern.

Further advice

The findings will be considered by the independent expert Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) at its next meeting in June.

It will decide if further advice needs to be given to members of the public.

Sue Davies, Principal Policy Adviser for Consumers' Association, welcomed the FSA decision to issue advice. She added: "The Food Standards Agency must ensure that the message gets through to high risk groups. It's appalling that, as a result of environmental contamination, some fish now contain toxic levels of chemicals, and can't be enjoyed by consumers."

Offficial figures suggest that 1506 tonnes of shark and swordfish were consumed in the UK in 2001 compared with 244,366 tonnes of cod and haddock, the most popular fish.

Medical experts suggest that people eat two portions of fish a week, one of which should be oily, as part of a balanced and varied diet.

Eating fish has been shown to significantly reduce the risk of heart attacks.