Warnings issued to pregnant women of the dangers of eating too much tuna or swordfish may be unwarranted, say scientists.
Swordfish: Back on the menu?
Research suggests that the amount of harmful mercury in these "predatory fish" may have been overestimated.
A scientist from the University of Saskatchewan in Canada says that the mercury may be stored in a less harmful form in the body of the fish.
UK experts say that more study is needed before the warning is scrapped.
Exposure to mercury has been linked with damage to the brain and nervous system - particularly in the developing foetus, where relatively small amounts could still be a danger.
Predatory fish such as tuna, marlin, shark and swordfish tend to accumulate in their tissues any mercury in their diet, leading to higher concentrations of the heavy metal than in other fish.
Until now, scientists had thought that the bulk of this was stored in the form of methyl mercury, which has the potential to be absorbed in human tissues quite easily.
However, the Canadian researchers have found, using a technique called x-ray absorption spectroscopy, that a great deal of the mercury stored in fish tissues is in a different form.
Rather than bound to hydrogen and carbon atoms, it has formed tight bonds with sulphur atoms, which, in principle, should make it far harder for the mercury to be taken up by the human body.
This means that it would be potentially less toxic than methyl mercury.
Dr Graham George, who carried out the study while based at Stanford University in the US, said: "People have used methyl mercury to model the toxic properties of mercury in fish because they don't know what's on the mercury in fish tissue.
"Now that we know this, we can better investigate its toxic properties."
He said that a drug to remove accumulations of mercury from human tissue was a possibility.
More research needed
However, Fiona Ford, a nutritionist with the WellBeing Eating for Pregnancy Helpline, said that it would be premature for mothers-to-be to disregard the precautionary warnings.
At present, women who are planning to conceive, pregnant or breastfeeding are advised to eat only one or two portions of these fish a week.
She said: "I would like to see this study repeated - we can't draw firm conclusions from the results of just one study.
"It would be very good if women were told that they could eat larger quantities of this type of fish.
"Tuna, in particular, is very affordable and contains large quantities of essential fatty acids which are very good for baby brain development."
The study was published in the journal Science.
The WellBeing helpline can be reached on 0845 130 3646 between 10am and 4pm Monday to Friday.