Millions of young women, and children under 16, were yesterday advised not to eat shark, swordfish or marlin because the fish might contain potentially dangerous levels of mercury.
Other adults should eat no more than one portion a week, said the government's food standards agency, which has also asked for a further opinion on tuna, although it believes consumers can continue to eat it safely.
The agency said it was issuing the warnings, which include women who intend to become pregnant as well as those who are, because of concerns that excessive amounts of methyl mercury might lead to development problems in the foetuses of pregnant mothers, in infants, and in fast-growing children.
Adults' nervous systems could also be affected, causing symptoms such as pricking and tingling sensations.
The advice could hit the burgeoning taste for more exotic fish, even though they, like the more humble (and less mercurial) cod, are fast approaching endangered status in the overfished oceans. Most Britons do not eat enough fish, which help prevent heart attacks, and any lingering concerns about tuna would be far more damaging to the catering industry and supermarkets.
Diana Bedford, a toxicologist with the agency, said: "If people eat one portion of shark, swordfish or marlin a week, they could be very close to the safety guideline established by the World Health Organisation for adults. If children ate one portion of these fish every week, they would exceed that safety margin."
The agency is seeking the advice of the independent committee on toxicity of chemicals in food, which meets next month, on whether further action is necessary. This might include ensuring importers, restaurants and supermarkets tested mercury levels.
Dr Bedford said: "In the meantime, we feel it is important to provide information to consumers on a precautionary basis so they can choose to avoid these fish if they want to."
The agency commissioned tests from Bristol University on a wide range of fresh, frozen and processed sea fish and shellfish. It said levels in other types did not give serious cause for concern, although it was seeking further guidance on tuna, since some samples tested almost as high as the mean 1.52mgs of mercury per kg of bodyweight resulting from the shark tests. Tuna is regularly eaten by Britons, although much of it in sandwiches would be the canned form where mercury levels are far lower.
Mercury exists normally in the environment as well as being caused by industrial pollution. The "danger" fish are harvested in many of the tropical and temperate parts of oceans and the agency was unable to say whether catches from some parts of the world had higher mercury levels than others.
The form of mercury found in fish results from conversion by bacteria. Predatory species at the top of the food chain, bigger and older than others when caught, tend to have higher levels. The agency's general advice is that adults should eat two portions of fish a week, one of oily fish, as part of a balanced diet. On average, UK consumers eat only three-quarters of a portion of white fish and a quarter of oily fish a week. Many fish fans, however, eat far more.
Waitrose, which sells marlin and swordfish, said it already insisted on tests by suppliers and was confident its fish were safe.