Food safety chiefs have updated their advice on eating tuna for pregnant or breastfeeding women.
The Food Standards Agency is advising them to limit their consumption to no more than two medium sized cans a week due to the presence of mercury in the fish.
Dr Andrew Wadge, acting director of food safety, said: "It's unlikely many pregnant or breastfeeding women eat more than the recommended amounts. But for any that currently do, it would be a sensible precaution to change their diets slightly."
Mercury, present in the fish, can cause harm to an unborn child's nervous system. The new advice does not apply to children or other adults.
The change in advice was sparked following a study by the independent Committee on Toxicity which discovered the levels of mercury within tuna was near the safe limit for pregnant and breastfeeding women recommended by the World Health Organisation.
A spokesman for leading tuna supplier John West said: "While nearly all fish contain some methyl mercury, canned tuna has only trace amounts. This advice is not new advice, and is similar to that issued by the US government over one year ago. We are confident the average consumer who eats canned tuna as part of a well balanced diet is at no risk from methyl mercury."