From the cutting edge of Tokyo and New York to the chilled cabinet at Boots, sushi is fast becoming the global convenience food of choice. But, Alex Renton asks, can raw fish and cold rice ever become the new Big Mac?

We don't have an address for Mr Sawada, the sushi master. Just a card with the samurai symbol of a red dragonfly and the name of a street off Tokyo's Ginza. We find the tiny dragonfly engraved beside a buzzer in an unremarkable doorway. 'This is a very extraordinary moment for me,' says Chie, our translator, as we troop up some shabby stairs. 'I could never eat here. I am not rich, I am not old enough.' She's in her mid-thirties; I think she means - 'not wise enough'.

Ordinarily you would pay some $500 in advance just to make a booking here at the table of one of Japan's most talked-about traditional sushi chefs. But then, we could never have made a booking, because Sawada serves at most eight people each mealtime and is booked up years ahead. He has given us a few minutes at the end of his day to photograph him in action.

We find a small square room made entirely - floor, ceiling, walls, chairs, counter and even the fridge - of pale lemon hinoki wood. This signifies luxury; it's used for the coffins of the emperors of Japan. There is no other colour in the room except a single pink camellia in a tiny vase on the counter.

The sushi master is solemn, shaven-headed like a Buddhist priest, and a dead ringer for Brian Cox. He shows us his knives, his charcoal stove, his rice cooker and his prep surface. His sushi is served plateless, on to the hinoki-wood counter, which is planed down after each meal until it is virgin again.

Sawada heaps rice straw on the charcoal burner, lights it and then passes a slab of bonito back and forth through the smoke. This is an ancient method, taken from the Tokyo Bay fishermen. When it's sufficiently scorched he takes the fish to the counter and cuts two finger-sized slivers from it. Beside him is a basket of warm, vinegared rice, Sawada takes a breath. He shapes his fingers into a position known as ninjitsu - aping Ninja fighters - and begins the gentle, rhythmic hand-jive that makes the nigiri sushi.

His palms move from side to side under his bent head, shaping a mini-loaf of rice. He smears it with a fingertip of wasabi. Then he lays the curved strip of lean fish-flesh over it, as though fitting a delicate piece of marquetry. He places the mouthful, precisely angled, on the counter. This is nigiri sushi, the original, unchanged in 180 years.

I have to ask him what he thinks of 'new sushi' - California roll, for example. He repeats the Japanese translation - kashu-maki - as though it's new to him. 'California roll? I find it - chaotic.'

A few months later, at 6pm in the Yo! Sushi shop in Haymarket, off Piccadilly Circus, a man is slicing a roll of raw red snapper, matter-of-fact even though he is - like all sushi chefs - on stage. The waxy flesh falls away from his knife like loam behind a plough. When he's done, using every last scrap, he starts on a beefy log of raw tuna. Shape, aim, cut, push. It's hypnotic. Soon, with some slices of bright orange farmed salmon, and a nest of shredded daikon radish and mustard cress, this will be a seven-slice Assorted Sashimi, yours for an amazing £5.

The sashimi goes on to the conveyor belt. Watching the bowls in their Habitat colours ride the track is just as monotonously mesmerising as watching the chef coddling his fish hunks: in front of the conveyor you turn train-spotter, wondering whether that piece of yellowtail and salmon roe will come round again.

The mechanical ballets of sushi and sashimi-making are an art whether you're in Tokyo, at Nobu, or at Yo! Sushi. There are differences, though. Sawada trained for six years, and at Nobu the chefs do three years before they are allowed to lay hands on the fish. (What do the novice chefs do for those three years? They wash the rice. They do the dishes. They watch and they meditate.) At Yo! the training is just two months. Across town at Nobu or at Zuma in Knightsbridge seven slices of purple, orange and white sashimi, not very different from Yo!'s Special Assorted, will cost you an easy £20, before service.

The origins of sushi are in fact more Yo! than Nobu or Sawada. It was a street food, a working-class dish - as so often, haute cuisine borrowed and tarted up a staple of the poor. Modern sushi - in the sense of raw fish served on vinegared rice - began at a street-food stall in the city of Edo, now Tokyo, in 1824. The stall's owner, Yohei Hanaya, was the first person to shape vinegared rice with his hands and then crown it with a slice of raw fish - prompted, it's said, by impatient customers, who couldn't be bothered to wait for the traditional pressing in a box.

Long before, at least 500 years before Christ, southeast Asians discovered that cooked rice begins to ferment, and that fish packed in this will be preserved. The technique is believed to have arrived in Japan in about 600AD. Around then in Europe we were learning that fish - for most people, the cheapest and the most readily available source of protein - could be made to last with vinegar or salt. Thus, starting with the same intentions, the Japanese got sushi. We got salt cod and pickled herring.

Hanaya's innovation made him sushi's Colonel Sanders. So popular were his nigiri (hand-rolled) sushi that nigiri-sushi stalls soon outnumbered most other food outlets in Tokyo. As with the sandwich - invented by an 18th-century Earl of Sandwich as handy food for eating when gambling - convenience was and remains the selling point. His rice fingers with their fish topping were and still are eaten usually by hand, upside-down, in two mouthfuls. Westerners have their own method with nigiri sushi - we take up the chopsticks and try hopelessly to scissor them in half. Then we may dunk the mouthful in a bowl of soy sauce until it disintegrates so we have to eat it grain by grain. Or we prod the whole nigiri into our mouths in one go - and choke. This is why many Japanese people would rather not watch Westerners eat sushi: it's not just gross, it's wrong.

Hanaya's sushi are recognisably what you'll find at Yo! Sushi, at Sainsbury's or even Boots. But there's debate about how much of Hanaya's fish was raw. Certainly some would have been marinated in soy or part-cooked, as prawn and eel usually are today. Raw sushi only became plentiful when refrigeration arrived in Japan in the 1930s. Salmon and salmon eggs aren't even listed among the contributing fish in my 1981 sushi cookbook - but now Japan imports nearly a-quarter-of-a-million tons of farmed salmon a year, approaching the levels for tuna.

What's now the most expensive cut of tuna, the fatty belly-meat of bluefin tuna called otoro and shimofuri, was cat-food until the 1950s: it became a delicacy as a result of a Japanese government marketing campaign to deal with tuna shortages caused, it's said, by nuclear weapons tests in the Pacific. Now, at Nobu, the melt-in-your-mouth piece of pinky-yellow shimofuri will cost you £6, compared with just £3 for a piece of lean akami off the top of the fish.

The next innovation was in the delivery. Not long after Ray Kroc of McDonald's was inspired by Detroit motor industry assembly-lines, Yoshiaki Shiraishi visited a beer-bottling factory in Osaka. He had run an under-performing sushi bar in the city. It didn't lack customers: his problem was the speed at which he could serve them. In retail economist-speak, he needed to up the foot-fall and lower the dwell-time. So he removed the chairs from his restaurant. That increased the customer numbers, but then he was faced with the problem of having to employ more staff to serve them. The bottling-factory conveyor belt was the answer.

Yoshiaki's first Genroku Sushi restaurants opened in 1958 and were an instant, profitable hit - especially after he put the customers at four-seater tables at right angles to the belt, instead of all facing it. That way four times as many customers got served. His chief problem was working out the right speed for the belt - too slow and the customers got impatient; too fast and there were accidents. And the air-flow was liable to dry the sushi. In the end, 8cm a second proved perfect.

In a few years Shiraishi had 240 conveyor-belt sushi restaurants; Japan now has 2,700 of them. (He died in 2001: he later lost most of his money trying to bury the profession of waitering for ever by developing sushi-serving robots.) When the World Fair was held in Osaka in 1970 Genroku Sushi set up a restaurant there - an exhibit that fixed the event in the minds of foreign commentators who were among the 64 million visitors.

Looking back, it's startling that those wide-eyed Western visitors didn't just grab the idea. But both raw fish and conveyor belts travelled slowly, and reached Europe and Britain last. We didn't get our first conveyor-belt sushi restaurant until Moshi Moshi opened in Waterloo station in 1994.

Sushi's migration and the process of its globalisation started in the large expat Japanese communities all round the Pacific rim - in western and southern America, Australia and Brazil. It became a rite of passage for Japanese chefs who'd been through the long sushi apprenticeship to go to these outposts, where the cuisine was less rigorously traditional. Nobuyuki Matsuhisa - now proprietor, in Nobu, of the poshest Asian cooking franchise of them all - made sushi in Lima, Buenos Aires and in Anchorage, Alaska before he settled in Los Angeles in 1978.

But the key moment in sushi's crossover from native cuisine to global snack is the invention of the California roll. It's the spaghetti bolognese of sushi now - a confection of cooked crab, avocado and mayonnaise inside a roll of sushi rice, probably bound with a strip of nori seaweed. It is an international cliché, but it was the crossover hit that took sushi out of Japan and on to the path to global dominance. If sushi were Abba, the California roll is Waterloo.

It is said to have been invented in 1973 at Los Angeles's Tokyo Kaikan restaurant (which still exists) by a chef called Ichiro Manashita. No one knows what happened to Manashita, but I have met Akihiro Izumi, now head chef at the JW Marriott in Bangkok, proud inventor of the New York roll. (Take the California roll, substitute apple - 'it's the Big Apple, right?' - for the avocado. Simple. There's a Philadelphia roll that does the same with cream cheese.)

Aki grew up poor in rural Japan, where sushi was a once-a-year family treat. After his five years training with a sushi master - 'the boss used to slap me round the head' - he left to work in a restaurant in Manhattan's Upper East Side. There they served traditional Japanese food to Japanese businessmen at a $50 minimum charge for sushi - 'and that was in the early 80s'.

After seven years, he went to California - 'just me, my sushi knife and my dog'. Aki found a new world. 'When I arrived I had no idea there existed a California roll. It was like a revolution in Japanese cooking. So many different kinds of sushi!'

This was liberating; he never went back to Japan. Now he works round the five-star hotels of Asia: he makes sushi with sautéed foie gras, beef, caviar and tempura, with balsamic or sherry vinegar in the rice. 'My sushi master would not approve,' he says.

Sushi's penetration happened slowest in Europe. Most English people who started eating sushi in the Eighties had that first, mind-opening bite in Manhattan, Sydney or San Francisco. London had Japanese restaurants but they chiefly fed the Japanese. Mark Edwards, now executive head chef of Nobu Group, and undoubtedly the greatest Japanese cook to emerge from Kent, lost his sushi virginity late - in 1987 in New York. His partner was a piece of otoro tuna belly served as nigiri. 'I was flabbergasted,' he says. 'I couldn't believe it. It was very exciting.'

He'd worked in restaurants in London, like the Café Royal, which served something they called sushi. 'We'd do it for canapés. But it was marinated fish dishes, or smoked salmon rolls. You have to understand, London was nowhere in the late Eighties. New York was the capital of new cuisine. And there was a consciousness that Asian was where things were headed.' Key to the slow arrival of raw fish on menus in London was the lack of materials. 'Even nine years ago, when we started Nobu in Park Lane, you'd struggle to get the ingredients. Now you can get anything.'

As Edwards started working in New York, an itinerant Japanese chef called Nobuyuki Matsuhisa was putting the savings from nine years labour at the LA sushi counters into a tiny new restaurant in Beverly Hills. Actors turned up, they liked drinking sake with Nobu. In 1989, Robert De Niro took his buddy the sushi chef to New York and showed him a loft. This was where they were going to sell New Yorkers Nobu's new Japanese cooking. Matsuhisa demurred, but de Niro kept badgering him. In 1994 they opened Manhattan's first Nobu - and, with that, sushi finally arrived in the fashion mainstream.

A combination of celebrity, surprising ideas and cool design made the tipping point. The formula was nothing new in restaurateering - it came out just right. The hook of Japanese food, especially raw fish, as healthy and non-fattening certainly helped. And Nobu flourished - there are 15 Nobus now across the world and a thousand copycats. The first London restaurant opened in February 1997, and another last year (both have Michelin stars). Nobu's dishes like black cod in miso and softshell crab roll are flagrantly, universally ripped-off.

The key to Nobu's success clearly lies in his willingness to compromise on the issue of raw fish - just as Manashita did with his California roll. Nobu's 'new-style sashimi' is the result, he says of a day when a customer returned a dish of fresh flounder, that Nobu had cut paper thin, arranged as you might the petals of a flower and served up raw with some shredded radish. As you would. But the customer would not eat raw fish. Some chefs would have thrown the yokel out of the restaurant, but Nobu had a thoughtful look around his kitchen. He reports: 'My gaze fell upon some olive oil that had been heating in a frying pan. "That's it," I thought. Sesame oil would be combined with heated olive oil to add aroma. The raw fish would be topped with ginger spears and menegi (shallots) and then drizzled with ponzu (a citric and soy sauce). Then the hot oil would be poured over it. The customer who had claimed not to be able to eat raw fish understood the way I felt. He tried a mouthful, then two, and ended up leaving the plate clean ... I am supremely happy at times like these.'

Nobu has every reason to be chuffed - this artistic unbending has been the basis of a multimillion-dollar business. And it was key to sushi's crossover. It led to the presence today in Boots the Chemist's sandwich fridge of a six-piece Shapers sushi roll pack at £2.30, maki filled with floppy red and yellow pepper slices, two with a sorry grey mash of cooked tuna and blanched salmon. The sushi masters would not like that at all.

The rawness was always the sticking point for Westerners. James Bond, the first tough guy in Western fiction who was also a foodie, enjoyed Japanese women and sake ('especially when it's served at exactly 98.4 degrees Fahrenheit', he quips geekishly in You Only Live Twice). But he didn't fancy sushi. As late as the Sixties a British guidebook to Japan could still warn of the 'stomach-turning' Japanese habit of consuming raw fish, which indicated fundamental differences between Oriental and Occidental digestions. Following advice given by the US Embassy, Americans in Japan during the 1950s ate teriyaki and tempura, but only consumed sushi if the fish was cooked.

Has much changed? 'Fifty per cent of the British population, when confronted with the idea of raw fish and cold rice, will go, "Yee-urrgggh!" That's what our research shows. We're not sure we can ever shift that.' So says Darren Wightman, group executive chef at Yo! Sushi, which is now Europe's biggest conveyor-belt sushi restaurant chain.

We're sitting in a concrete bunker in Yo!'s head office in London's Farringdon. The war-room look is not inappropriate. Yo! has fought and won the battle to dominate the British 'fast-casual' sushi market. Now, with branches already in Dubai and Paris, it is planning an assault via franchisees on the United States. We're talking big - sushi is the 'Japanese tapas', sushi is the new pizza, sushi is the sandwich of the 21st century. It's healthy, Zen, stylish.

But it has a long way to go. Georgia Hall, Yo!'s feisty head of marketing, suddenly admits that she ate her first sushi only a year ago, shortly after she joined the company. We all look amazed - Darren particularly. An Australian, he ate his first as a teenager. Why so long? 'Well, I'm a Geordie. People in Newcastle don't eat raw fish. They eat pie and chips.'

Yo! is pushing hard at that 'immature market'. Its spread of 21 restaurants in the UK (with seven more on the way) crosses several class divides. You can eat Yo! sushi in Bluewater and in Selfridges, in Paddington Station and in Mayfair. The branch in London's Haymarket, off Piccadilly - a classic 'high foot-fall' area for fast-turnover food - is on a site where a McDonald's branch failed. While Yo! has had its own failures - an Edinburgh branch closed, and Newcastle is a nut they're not going to crack yet - clearly it has pushed the notion of raw fish deep into the conservative part of a nation that previously liked its fish white and in a coat of batter or crumbs.

Yo! was likeable but chaotic when Simon Woodroffe launched it in 1997: now under more professional management it is a very hot brand. It is what the catering industry calls a 'category killer' - capable of knocking out all competitors in its market segment. Think Wagamama, Pizza Express and La Tasca. By the middle of 2005 Yo! already had 45 per cent of the 'fast-casual' sushi market - a million customers a year. In America, they think, there's potential for 4,500 Yo! Sushis. 'No one there has yet done converyor belt sushi sexed-up - it's all bottom-end, drab and boring,' says Georgia.

Conveyor belts are crucial to Yo! - they're not just the gimmick, they're also key to profit and product turnover, explains Darren Wightman. Without the conveyor belt, revenues drop 35 per cent. The conveyor belt means that dwell-time - the number of minutes customers are in the shop - can be as low as 12 minutes, during which a customer will spend, on average, £14. Pizza Express takes twice as long to extract that much cash. In Paddington Station Yo!'s mere 15 seats will cater to as many as 400 covers a day. And a combination of wily head chefs and computer tracking keeps wastage to mere 1.5 per cent - in this sort of catering 5 to 6 per cent is normal.

These sort of figures enchant investors. But what's impressive for the Yo! customer is that the food on the track is no more expensive than five years ago - the price range of the coloured plates is still between £1.50 and £5 - and it is hugely improved. The original Yo!s served a choice of 40 or so dishes. Today there are 110 to 120 on the track or immediately available and 400 'in the repertoire'.

And quality has gone from dubious - in the original wacky, manga-themed Yo! shops of the late Nineties - to extremely good. Tellingly, they use the same fish supplier as Nobu. Mark Edwards of Nobu is a Yo! fan - 'I like it - I would eat there: they keep costs down and the quality good, and I respect that.' Colin Clague at Zuma, London's brightest new-style Japanese restaurant, says he likes Yo!, but that, when you put any sort of mechanisation into the process, what you're producing isn't really sushi any more.

So where is sushi going? Clearly fast-food sushi is going to boom. Pret A Manger - which started selling chilled sushi from its shops the same year that Nobu opened in London - now shifts 25,000 to 30,000 of its £4.95 'Sushi deluxe' packs a week - about the same as for tuna mayonnaise or chicken-and-avocado sandwiches. And Pret A Manger is now part-owned by McDonald's. Sushi is the new sandwich. In the fast-food world where customers are ever more neophiliac, it's still seen as 'healthy' and 'innovative' - the two key adjectives that, according to Pret's commercial director Simon Hargraves, made it attractive for the chain nine years ago.

At the top end, the simple and ancient art of the sushi master is still under assault from the forces of revolution. The sushi competition run each year by Matsuri in London's High Holborn was won last year by a mozzarella, spring onion and almond confection (the runner-up was mashed kipper and crispy-bacon sushi).

Mark Edwards has spotted a sushi risotto in Milan; in Bangkok I had a fright at 'pizza sushi'. In Japan the endless search for obscurer marine creatures to eat continues. There's a tradition in the country that each new thing you eat adds a year to your life, which may account for the kamachi kombu Edwards once ate - a frond of dried seaweed with the eggs laid there by a herring still attached.

Having survived 2,500 years of changes, sushi faces its greatest challenge just as it goes global: the catastrophic decline of the basic raw material. Almost every staple sushi and sashimi fish is threatened in the wild, most of all the bluefin tuna. Farmed fish provide some answers - thus the rise of the use of salmon - but farmed fish have to be fed on fish, and this has caused declines in other stocks.

Real sushi will in the end be only for the rich. Street sushi will increasingly feature what the industry unappetisingly calls 'seafood analogues' - imitation fish. The technique of surimi - blanching and processing fish protein - is another ancient Japanese invention: but today with the addition of vegetable starches, sugar, flavouring and colouring it brings us crabsticks, 'shredded shellfish', faux-lobster tails and clams, and even reconstituted fish steaks. America produces 150,000 tonnes of it a year and Alaskan pollack, the principal source of fish-processing protein, is now threatened by over-fishing.

But as the techniques improve, customers are being seduced.

Akihiro Izumi in Bangkok told me customers demand crabsticks - 'they prefer it to real crab'. He thinks this is blasphemy, but I say: keep it coming. Mock fish means more bluefin otoro moments for the rest of us.