This interim advice is being issued as a precautionary step following a Food Standards Agency survey that revealed relatively high levels of mercury in these species of fish.
Large predatory fish such as shark, swordfish, and marlin can contain relatively high levels of mercury in the form of methylmercury. This can harm the nervous system of an unborn child if the fish is eaten regularly by its mother.
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.
Occasional consumption of shark, swordfish, or marlin as part of a balanced diet by any other adults is unlikely to result in harmful effects.
| Methylmercury found at high levels in shark, swordfish, and marlin.
However, on a precautionary basis, they are advised against eating more than 1 portion each week of either shark or swordfish or marlin.
The Food Standards Agency surveyed 336 fresh, frozen, and processed sea fish and shellfish for mercury content. These included trout, salmon, tuna, halibut, hoki, seabass, lobster, mussels, and prawns.
Levels of mercury in fish other than shark, swordfish, and marlin did not give cause for concern.
Previous UK surveys have not found mercury at levels that cause concern in the UK's most frequently consumed fish - cod, haddock, and plaice.
1506 tons of shark and swordfish were consumed in the UK in 2001, compared with 244,366 tons of cod and haddock.
The independent expert Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) will consider the survey results and any implications for consumers at its June meeting.
When the COT has completed its considerations, any further advice will be issued if necessary.