The researchers are to appeal to the Food Standards Agency and the Government's Scientific Advisory Committee on Nutrition in an open letter asking the two bodies to revise their advice.

They believe that women should be eating at least three portions of fish a week.

Fish consumption among women of child bearing age and girls is currently restricted due to concerns about the presence of potentially harmful contaminants in fish such as dioxins and methylmercury.

Speaking at a conference held at the Royal Society of Medicine in London last week, Professor Jack Winkler, director of the Nutrition Policy Unit at London Metropolitan University, said the benefits of fish oil far outweighed other risks.

He said: "The Food Standards Agency issued advice to women and pregnant women about fish in 2004 which was ultra conservative. Since then, there has been research which indicates that women who have eaten more fish than those recommendations suffer no harm but their child's brain performance improved.

"The evidence is beginning to show that this ultra conservative advice is effectively denying women the benefits of fish. Worryingly the current advice is scaring women off eating fish completely as the message it gives is that fish is risky."

A study published in the Lancet in 2007 of almost 12,000 pregnant women showed that those who ate less than 340 grams of seafood a week, which is equivalent to two and a half portions, had children who were at greater risk of having low verbal intelligence.

It concluded that the risks from loss of nutrients were greater than the risk of harm from contaminants in fish.

Professor Michael Crawford, director of the Institute of Brain Chemistry and Human Nutrition at London Metropolitan University, said: "Unlike the rest of the body, the brain is mainly made of fat. It needs these fatty acids for brain growth and development.

"We are deeply concerned that this has been more or less neglected in the current advice and unless there is a change in nutrition advice to take the brain into account, then mental disorders are going to continue to grow at an alarming rate."

New research presented at the conference also suggested that docosahexaenoic acid deficiency may also play a role in the development of behavioural disorders such as ADHD in children.

A study by Dr Robert McNamara, from the department of psychiatry at University of Cincinnati College of Medicine, found that boys aged between eight and ten who were given additional docosahexaenoic acid had increased brain activity in attention tasks than those taken placebos.

Fish oil has also been linked to a number of other health benefits including reducing the risk of heart disease and cancer, combating memory loss and arthritis. The sale of supplements containing omega 3 fatty acids is now a major business.

Some recent studies have also found no evidence that taking fish oil tablets had any effect on boosting the academic abilities of children.

A spokesman for the Food Standards Agency said: "Our advice on oily fish consumption in pregnancy is based on a 2004 review involving two independent scientific committees who weighed up the nutritional benefits of oily fish against the possible risks, and the report included pregnant and lactating women."