But they warn against eating certain types of fish, such as shark and marlin - or lots of tuna - because of the risks to the developing foetus associated with mercury.

Previous research from the Avon Longitudinal Study Group has shown that omega-3 fatty acids contained in fish - particularly oily fish - are associated with boosting children's future brain power and social skills.

However, the team had only looked at the effects up until the age of three or four.

Behaviour

In the new analysis, researchers from Bristol University and the US National Institutes of Health questioned 11,875 pregnant women on their fish and seafood consumption.

They looked at social and communication skills as well as hand-eye co-ordination and total IQ in the children up to the age of eight years.

Socioeconomic factors were taken into account as well as information on the rest of the women's diet.

Eating less than 12oz (340g) of fish and seafood a week was associated with a 48% increased risk of children being in the lowest group for verbal intelligence.

Low fish and seafood intake during pregnancy was also associated with increased risk of poorer behaviour, motor, communication and social development scores.

The lower the consumption of fish, the higher the risk of poorer scores on the neurodevelopmental tests, said the researchers.

Professor Jean Golding, emeritus professor of paediatric and perinatal Epidemiology at the University of Bristol, and head of the study, said women should follow the FSA advice and eat a mixture of different types of fish.

Oily fish, the most widely available source of omega-3 fatty acids, that women should eat include salmon, mackerel, pilchards and sardines.

She said: "The findings we had previously were very much earlier in a child's life so it was quite possible the effect would have worn off.

"But here we have very convincing findings up to the age of seven or eight."

Foetal exposure

Professor Robert Grimble, professor of nutrition at the University of Southampton said omega-3 fatty acids were very important for brain development.

"This idea of fish being toxic has been around for a long time but this study seems to be saying that is a minor problem compared with the benefits you get from fish."

"Studies have shown improvement in brain ability and reduction in antisocial behaviour."

He added that infants would get omega-3 through the placenta when they were in the womb but also through breast milk once they were born.

In 2004 the US government issued advice to women on limiting their intake of overall seafood and fish to 340g per week to avoid foetal exposure to trace amounts of neurotoxins.

In the UK, pregnant women are advised not to eat more than two tuna steaks a week (weighing about 140g cooked or 170g raw) or four medium-size cans of tuna a week (with a drained weight of about 140g per can) because of the levels of mercury.