The Vegetarian and Vegan Foundation says that pollutants such as dioxins and polychlorinated biphenyls (PCBs) can accumulate in the fatty parts of fish which are then used to manufacture fish oil supplements.
A report, Fishing for Facts, published by the Foundation in 2003, says the adverse effects of PCBs and dioxins 'may include cancers, damage to the immune system, infertility and birth defects'.
Dr Justine Butler, molecular biologist and health campaigner for the Foundation, said, 'Most of the world's oceans are polluted and we think it is dangerous to promote the consumption of fish and fish oil supplements.
'Plant oil supplements, such as flaxseed oil, are a good alternative source of omega-3 fatty acids. We would recommend that parents refrain from giving their children fish and move towards a plant-based diet.'
Dr Butler plans to express her concerns to co-ordinators of the Durham Schools and Durham Sure Start trials, which have examined the effects of supplements on young children.
Children participating in the trials were given daily doses of the 'eye-q'
supplement from manufacturer Equazen. Adam Kelliher, managing director of Equazen, said, 'I am deeply concerned about blanket assertions that fish oil supplements are contaminated with environmental toxins. All of Equazen's fish oils are in complete compliance with stringent guidelines laid down by the World Health Organisation and the European Union.'
Mr Kelliher added that recent evidence showed plant oils were not as effective at promoting brain function as fish oils - an assertion supported by the Food Standards Agency (FSA).
The FSA recommends that children eat two servings of fish a week, including one of oily fish. A spokesman said, 'It is better for children to eat fish than to take supplements because, as well as containing omega-3 fatty acids, fish is an excellent source of other nutrients.
'Medical advice should be sought from a GP before giving any supplements to children. Instructions on the label should always be followed and the supplement should be suitable for the child's age group.'
WHO requires supplement manufacturers to monitor the levels of seven types of dioxins and 17 different PCBs.