The offending samples included bluefin tuna, the hugely-prized species that has been plunged into a fierce conservation battle, the researchers reported in the British journal Biology Letters.

One hundred sushi samples were collected from 54 restaurants and 15 supermarkets in New York, New Jersey and Colorado, comprising "akami" (lean red tuna) as well as "toro" (fatty tuna).

The species were identified using a DNA fingerprint test and the samples were then tested for mercury levels.

A leading benchmark for safety is a maximum daily consumption of 0.1 microgrammes of mercury per kilo of human bodyweight per day, set by the US Environmental Protection Agency (EPA).

Calculated on the basis of a 60-kilo (132-pound) adult woman consuming a single order, samples of Bigeye tuna toro were found to have average mercury levels of 0.351 microgrammes per kilo, while Bigeye tuna akami had 0.344 microgrammes.

Bluefin toro samples had the equivalent of 0.123 microgrammes per kilo of bodyweight per day, and bluefin akami 0.180.

Yellowfin tuna, found in the samples only as akami, had 0.164 microgrammes of mercury per kilo of bodyweight.

"The mean mercury concentrations of all samples exceed the concentration permitted by Japan and the maximum daily consumption considered safe by the US Environmental Protection Agency," said the paper.

"Mean mercury levels for bluefin akami exceed those permitted by the US Food and Drug Administration (FDA), Health Canada and the European Commission.

"On average, one order of Bigeye tuna sushi, the species used most often for sushi, exceeds the safe maximum daily dose recommended by Health Canada and the safe limit established by the World Health Organisation (WHO) and FAO [Food and Agricultural Organisation] for women of childbearing age."

The study noted that mercury levels were higher in tuna sushi sold in restaurants rather than in supermarkets.

This was because supermarket samples were far likelier to be yellowfin tuna, the species with the lowest mercury contamination.

The study, led by Jacob Lowenstein at the American Museum of Natural History in New York, breaks new ground.

DNA fingerprinting -- identifying genetic tags that are exclusive to a given species -- has been used until now to help trafficking in endangered wildlife.

It is the first time that the technique has also been used in health research.

The authors say the findings are useful for people worried about ingestion of mercury, a toxic chemical that is passed up the food chain, progressively accumulating in larger carnivores.

Excessive mercury is linked especially to neurodevelopmental defects, including mental retardation, cerebral palsy, deafness and blindness.

Many countries have issued advisories notifying consumers about fish that are high in mercury, yet at the same time allow poor labelling of fish species, the paper says.

Bluefin tuna is considered a delicacy in Japan, where a single 220-kilo (485-pound) fish can fetch 160,000 dollars (120,000 euros) at auction.

Japan fought fiercely at a conference in Qatar last month of the Convention on the International Trade in Endangered Species (CITES) to thwart proposed international trade in bluefin caught in the Mediterranean and the eastern Atlantic.

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