The UK Food Standards Agency is concerned that mercury found in the fish could pose a health hazard.

It says women who intend to get pregnant should also be careful about eating too much of the fish.

When planning to have a baby and while pregnant or breastfeeding, women do need to take particular care of their health and that of their baby Dr Andrew Wadge

The FSA says all these women should not eat more than two medium size cans of tuna a week.

They have already been advised to avoid any shark, swordfish and marlin.

The new advice about tuna does not apply to children or any other adults.

A survey conducted last year found levels of mercury in a wide range of commonly eaten fish.

The latest advice follows a review of the results by the independent Committee on Toxicity, which advises the government.

It found the amount of mercury in tuna was near the limit it sets for pregnant and breastfeeding women and those considering having a baby.

The FSA said there was a "small risk" to unborn children and new babies because mercury can harm their nervous system.

Pollution

Mercury is released naturally into the environment through the earth's crust into the ocean but is also absorbed by the sea as pollution from burning household and industrial waste.

Dr Andrew Wadge, acting director of food safety at the FSA said: "It is unlikely that many pregnant or breastfeeding women eat more than the recommended amounts of these fish every week.

"But for any that currently do, it would be a sensible precaution to change their diets slightly.

"This will help protect the unborn child and the developing breastfed baby.

"When planning to have a baby and while pregnant or breastfeeding, women do need to take particular care of their health and that of their baby."

Two medium size cans of tuna equates to approximately six rounds of sandwiches or three tuna salads a week.