The shaven-headed 'sushi master' bends over a choice cut of raw tuna. His head cocks slightly to one side as he sizes up the moist, firm flesh. After a moment's meditation, he slices up the tuna into a dozen bite-size morsels with a series of swift knife strokes.
The slivers of fish are then laid lovingly atop small mounds of fragrant Oriental rice smeared with hot wasabi sauce. Simple, clean and fresh, sushi is the epitome of 21st century eating. Delicate fish, rich in essential oils, and nori seaweed loaded with minerals, are helping sushi to become the definitive lunch option for health conscious 20 and 30-something Britons.
Such is sushi's growth in popularity that it's now outselling the traditional BLT sandwich in many places. It is fast becoming a national staple alongside ploughman's, pizza and curry. But as the young and upwardly mobile tuck in, they would undoubtedly be horrified to learn what lies behind the neat little packets of rice and fish, wrapped in seaweed.
The Mail can reveal that far from being a healthy alternative to the sandwich, sushi contains a cocktail of chemicals, heavy metals and pesticides which can potentially lower intelligence, reduce fertility and even lead to cancer. They are also, despite their healthy image, laden with calories
Rising sushi consumption is also leading to the destruction of some of the world's last great sea fisheries, as well as helping despoil the pristine lochs of western Scotland.
"If you eat a meal of salmon sushi more than twice a year, you will increase your risk of cancer," says Professor David Carpenter, an environmental health scientist at the University at Albany, New York.
"The contaminants found in fish often overpower its beneficial effects. People think they're improving their health by eating sushi but they are in fact poisoning themselves."
Sushi is a simple food made from rice steeped in vinegar and topped with a variety of fish - most commonly in Britain, raw tuna, prawn and salmon - called nigiri sushi.
Its staple ingredients are, most commonly, rolled up inside a wrap of nori seaweed to produce the Oriental equivalent of the sausage roll. These rolls - called maki or California rolls - are then sliced into bite-size pieces, allowing them to be eaten with chopsticks, and they're becoming increasingly popular in Britain. They are sold with nigiri sushi in packs known as 'bento' lunchboxes. The dinky little packages contain chopsticks, a tiny bottle of soy sauce, a heap of pickled ginger and a blob of hot wasabi sauce.
But a single California roll, containing crabstick and avocado, can easily contain 400 calories and 2g of salt
. Many lunch boxes contain several of these mini rolls, so it's easy to over-indulge on carbs, fat and salt without realising it.
Compared to the quintessentially British egg and cress sandwich, which has around 250 calories, a sushi box is hardly a slimmer's delight.
According to Professor Tim Lang, a food policy expert at City University in London: "The problem arises because we're trying to bolt sushi on to our national diet, which is full of highly processed food already high in salt and fat. Sushi comes from a culture that is inherently healthy. You can't just add sushi to our way of life and expect to get the same benefits."
Nonetheless, cashing in on the ever-increasing desire for healthy food, sushi bars are springing up and exploit the fashionable desire for people to see their food lovingly prepared - in this case - in front of their eyes by dextrous chefs. But, like the bento boxes, these restaurants are only the final link in a long chain that begins in the polluted salmon lochs of Scotland and the filthy seas of southern Europe and the Far East
The stench of old fish and diesel fills the air in a busy fish processing plant in southern Spain. The incessant hum of machinery is so powerful you feel they might shake your teeth loose.
Every half hour or so a truck roars into the factory laden with tuna from the warm, yet polluted, waters of the Mediterranean. Dozens of workers clad in white overalls rush to attention and start toiling alongside conveyor belts liberally covered in tuna blood.
Each worker grabs a 5ftlong fish and slices open its pinkish belly before reaching inside and ripping out its innards. Within a few minutes, the tuna has been cleaved, beheaded and 'flash frozen' into rectangular blocks, ready for the sushi bars and supermarkets of Britain, America - and Japan.
In the intensity of the processing factory, the workers can be forgiven for overlooking the odd fish infested with parasitic worms
. Spotting their tiny larvae is even more difficult, if not impossible, so the tuna bound for supermarkets is frozen for 24 hours at minus 20C to kill off the worms.
However, if diners are sensitive to worms - as many people are - they react to the dead worms in the tuna
. Initial queasiness will swiftly lead to severe stomach pain and vomiting which will last for a few days.
There is also fish - particularly that which is prepared in small restaurants - that is not frozen and still contains live worms.
Eating this can cause serious intestinal problems. In extreme cases, the worms will trigger anaphylaxis, a potentially lethal reaction similar to that suffered by people with nut allergies. Vicious bugs such as salmonella and even typhoid can also be passed on through sushi, largely because the flesh is eaten raw.
However, in the long term, these bugs and worms are likely to be the least of your worries. Tuna and salmon are loaded with mercury and a mix of nasty industrial chemicals such as dioxins, pesticides and PCBs, which have been dumped in our seas and oceans.
And once eaten, these poisons stay in the body for decades, reducing fertility and steadily weakening the immune system and potentially causing cancer.
Professor David Carpenter and his team at the prestigious Universities of Cornell, Indiana and Albany, recently studied the levels of these poisons in salmon fished from waters around the world. His work makes for uneasy reading.
Out of the 15 poisons detected in frighteningly high amounts, 13 are carcinogenic.
These poisons have also been linked to falling sperm counts, rising birth abnormalities, testicular and breast cancer, endometriosis and early puberty.
And if all that wasn't enough, some scientists worry that they may be acting as 'gender benders' by making young boys more feminine and girls more masculine, which may also affect sexual orientation later in life.
The scientists also found worrying levels of these pollutants in farmed salmon, especially those from Scotland. Professor Carpenter's team was so shocked by the findings of their study, which remains the biggest and most comprehensive so far, that they recommended people eat farmed salmon at most twice a year.
His findings are brushed off by the Scottish Salmon Producers' Organisation (SSPO). Spokesman Ken Hughes says: "The health benefits of eating oil-rich fish, such as salmon, outweigh any potential risks from contaminants that are ubiquitous in the environment."
Like salmon, tuna sushi is often touted for its healthiness because it is rich in the omega three fatty acids. But it is also loaded with a particularly toxic form of mercury.
An analysis of sushi from restaurants in Los Angeles in February, found dangerous levels of mercury in a quarter of the samples tested. Three quarters were above limits set by the Food Standards Agency (FSA) and its US equivalent, which permits one thousandth of a gram of mercury per kilo of fish.
This may not sound like a lot but it is two and a half times the amount permitted by the Japanese government because mercury has a cumulative effect and can build up in the body.
The FSA advises pregnant women not to eat shark, swordfish or marlin - and to eat only two tuna steaks a week - because of the high levels of mercury found in these fish.
Although no one has analysed mercury levels in sushi in the UK, given that tuna is traded internationally, it is reasonable to suppose that they would be much the same as in LA.
In adults, the most common effect of methyl mercury poisoning is paresthesia, a sensation of prickling or tingling on the skin. People may also feel sick and generally off-colour. Children - and foetuses - are particularly vulnerable. Mercury can reduce their intelligence and lead to behavioural problems.
Disturbing though Professor Carpenter's findings are for humans, the environment may be paying an even higher price for our new found love of sushi.
Every year, thousands of dolphins turtles, sharks and seabirds drown in tuna nets. Several species of tuna have already been driven to the edge of extinction.
Salmon farming can cause massive environmental problems, releasing chemical pollutants into the sea and consuming vast amounts of 'fish chow' - made from young fish caught at sea - leaving bigger fish such as cod, herring and mackerel to go hungry.
All of these problems have led some experts to conclude that the pleasures gained from eating sushi come at too high a cost.