It can reduce the risk of heart attacks and improve circulation.

But fish such as mackerel, salmon and swordfish may contain high levels of potentially carcinogenic chemicals, and others, including shark, marlin and swordfish contain high levels of mercury.

A survey by Which? magazine found only a sixth of fish eaters knew they should only eat oily fish once a week.

More than half thought they should eat two or more portions.

Oily fish

Herring Kippers Mackeral Marlin Pilchards Salmon Sardines Sprats Swordfish Trout Fresh (not tinned) tuna

In fact, the advice is to eat two portions of fish per week, one of which should be oily.

Two thirds of the 970 people surveyed were unaware oily fish could contain potentially harmful chemicals.

Long-term exposure to PCBs and dioxins in animals has been shown to cause damage to the immune and reproductive systems and may cause cancer in humans.

Mercury

Eating more than one portion of oily fish a week means someone is more likely to exceed the tolerable daily intake for dioxins and PCBs.

Which? says this will not automatically result in a health risk, and it is the overall intake over long periods of time that matters.

As part of a balanced and varied diet, people should eat at least two portions of fish a week, one of which should be oily

Food Standards Agency spokeswoman

Just 7% of those surveyed knew about recent advice from the Food Standards Agency that pregnant women, women who intend to become pregnant, infants and children under 16 should avoid fish which could have high mercury levels, but none knew the details.

Only 1% knew pregnant women should avoid certain types of fish because of mercury levels, but no-one could name the fish.

Many people did not even know which kinds of fish were classed as oily, with 14% wrongly identifying cod as an oily fish.

Sardines and mackerel were the only ones correctly identified by most people.

Which? criticises the FSA's advice as "vague and difficult to follow".

It questions how big the recommended one portion a week should be.

Contamination

Helen Parker, editor of Which? said: "The FSA needs to ensure its advice reaches the people who need to know.

"Pregnant women and those planning to become pregnant are priority groups.

"And the advice itself must be clear and actionable - explaining exactly which fish are involved, what a portion means, and the risk of exceeding the guidelines."

She added: "We welcome the news that the FSA is bringing together experts to review the benefits and risks of oily fish, and we hope its future advice will be more detailed and easier to follow.

"In the meantime, we'd recommend that consumers eat oily fish no more than once a week, and remember that fish oil supplements - such as cod liver oil - may be contaminated too."

A spokeswoman for the FSA told BBC News Online: "We welcome this survey and will be studying its findings carefully.

"What we advise is that, as part of a balanced and varied diet, people should eat at least two portions of fish a week, one of which should be oily.

"We have however, been asked about the risks and benefits of eating larger amounts of oily fish and are seeking the advice of our experts on this.

"At present though, most people eat considerably less fish that we recommend."

She said the FSA was constantly looking at ways of improving how it provided information to the public.

Follow recommendations

Sara Stanner of the British Nutrition Foundation told BBC News Online: "We agree that there is good evidence for health benefits of n-3 fatty acids, particularly in relation to cardiovascular disease which is the biggest killer in the UK.

"The FSA recommend that people eat one portion of oily fish per week.

"If people do follow this recommendation their intake of dioxins and other contaminants should not be at a level to confer a health risk."