The Food Standards Agency (FSA) warned pregnant women two years ago to limit their consumption of tuna to two cans a week to protect their unborn children.

There are now concerns — echoing health fears in America — that children should be prevented from eating too much tuna because its flesh becomes contaminated more than most fish with mercury from polluted waters.

An FSA spokesman said this weekend that it would fund research to improve testing for mercury in fish.

Concerns have been raised following the case of an American child who developed learning difficulties after eating a portion of tuna a day. Doctors who conducted blood tests concluded the child was suffering from mercury poisoning.

Michael Bender, director of the Mercury Policy Project, a non-profit organisation that campaigns for reduced mercury pollution, said: “The FSA needs to take the next step and recommend a proper limit for the amount of tuna children can eat.”

Mercury occurs naturally in the environment, but is also released into the air through industrial pollution which is taken into the sea by rain. Fish absorb it as they feed, but some larger predators such as tuna are more likely to have higher concentrations because they feed on smaller fish.

Concern over the potential risk was highlighted in Britain after a study in the Faroe Islands in 1997. It concluded that children exposed in the womb to mercury had difficulties with learning, memory and attention.

Although the fishing industry argued the study was flawed, the Environmental Protection Agency (EPA) in America subsequently set recommended safety limits for the amount of mercury in adults’ and children’s diets.

In Britain the FSA advised children not to eat shark, marlin and swordfish. Tuna has, however, been exempt from restrictions because tests showed it had lower mercury levels. On the other hand, it is a fish that is popular with children.

The FSA said last week there was “no upper limit” for children eating canned tuna. Campaigners, however, say the FSA should now give parents guidelines on the amount of tuna children can eat.

In California it has emerged that Matthew Davis, 10, consumed a portion a day of white albacore tuna. Although he had previously seemed bright and motivated, he started to have learning difficulties.

His parents became concerned when they noticed his fingers were starting to curl, as if he was holding a ball. A neurologist ordered blood tests and it was found he was suffering from mercury poisoning.

An FSA spokesman said last week that only a small quantity of tuna in Britain was albacore. He added the agency was recommending to the European Union that it funded research to establish the potential risk.