Research by the Ministry of Agriculture and the Department of Health found low levels of dioxins and PCBs in all brands of fish fingers bought in supermarkets.

Dioxins are industrial by-products that also occur naturally and have been linked to cancer and while PCBS are thought to be responsible for so-called gender bending effects because they mimic the female sex hormone oestrogen.

Even greater levels of contamination were found in fresh fish from London's Billingsgate and Aberdeen fish markets, with the highest in oily fish such as herring.

Fish, chips and a side order of deadly chemicals?

Ignoring the advice to eat less fish could lead to an excessive accumulation of the poisons building up in people's bodies.

The findings add to growing concern about the state of Britain's food, just days after another scare about high levels of pesticides in vegetables, and recent warnings about antibiotics in meat.

It will disturb those who have switched to eating fish believing it to be healthier than meat.

Although levels of the chemicals were generally below maximum intakes recommended by the World Health Organisation (WHO), the new warning was prompted by fears of cumulative effects.

Children most vulnerable

Yet no advice was issued when the report was published last week.

A spokesman for the Ministry of Agriculture defended that decision: "The recommendation made to us was that because the levels of chemicals found were below those perceived to be dangerous, no warning was required."

According to the WHO children are already exposed to levels of the two toxins that exceed its safety limits, which puts them most at risk from polluted fish.