It might not be the world's first hybrid of an allotment and a fish farm, but it is almost certainly the first to be established on Dalston Lane, north London. Step inside the front door of what was once an ordinary shop, 100 metres from Dalston Junction Station in Hackney, and you will be greeted by a sight that is novel even by the standards of the capital's most diverse and unpredictable borough. As buses and lorries shuffle past on the other side of the wall, 80 Nile tilapia gaze, mouths agape, into a whitewashed room lined with racks of herbs and vegetables, which feed on the nutrients in the fishes' waste-enriched water and return it, cleaned and filtered, to the tanks.

The fish won't be ready for eating until August, but in the meantime the café in the room next door is kept supplied with herbs, salad leaves and other produce grown in their waste. "Even if you weren't going to eat the fish, there would be a case for running things like this just for the vegetables you can produce," says Paul Smyth, a member of the design practice Something & Son, which set up the "aquaponics" system in March 2010.

As its name implies, Farm:shop is an attempt to turn the formerly empty building into a working urban farm. Other projects under way in the four-storey space include rearing chickens in a coop on the roof and growing vegetables in a polytunnel in the back garden. Smyth says that the farm is planning to grow mushrooms in the basement and has set aside a space for pigs. Apart from cows, which he concedes "are better off where they are", there are few aspects of the conventional farmer's repertoire that he and his partners have not considered re-creating on Dalston Lane. "If you get a big warehouse or a rooftop, you can grow food like people do in the countryside," he says optimistically. "There's no reason why that kind of thing can't happen in London."

Ever greater numbers of people seem to agree. Farm:shop is listed as number 599 in the 2,012 new food-growing spaces that the Mayor of London's Capital Growth campaign hopes to be supporting by the end of this year. Since it was included in the list last summer, more than 400 other spaces have been added. New sources of food production have been established in the most improbable places, including the former railway lands behind King's Cross, where a company called Global Generation has devised a series of temporary gardens, one of which is made up of six skips that can be moved from vacant plot to vacant plot as the redevelopment of the 67-acre site proceeds.

Paul Richens, gardens manager of Global Generation, built a garden on the roof of a car park in Croydon in 2008 and has established another above the offices of Wolff Olins, a brand management agency a few hundred metres east of York Way, using a technique he calls "snowshoe rafting" to spread the load. When I went up there, the wooden beds and woven baskets were filled with currant bushes, celery, broad beans, potatoes and a healthy-looking crop of cabbages, which, according to Richens, do well thanks to the "wide sky eye" that ensures a profusion of light.

Some of the produce will be cooked in the canteen on the ground floor; some will be sold to local businesses. The food-growing, however, is not an end in itself. Global Generation says that it is "dedicated to giving young people opportunities to play a part in creating a sustainable future", and local children are involved in every aspect of their schemes, from designing the gardens and growing the food to selling the produce. "It's a ticket for places they wouldn't normally have access to," says the company's executive director, Jane Riddiford.

Food-growing schemes allow local schoolchildren to develop a "deeper connection" to the rapidly changing neighbourhood, Riddiford says, and others claim similar effects in areas less dynamic than King's Cross. Rosie Boycott, chair of the London Food Board, which has championed the Capital Growth campaign to promote its aim of providing residents of the city with "healthy, locally produced and affordable food", says that run-down estates have been transformed by the creation of food-growing spaces.

“You put a garden in, find someone who will lead that garden and reclaim the space. The feral youth vanish and it has a remarkable bonding effect," she tells me. "It's the antithesis of the me, me, me generation - there's an incredible pride in doing something for nothing. The beauty of it is that everyone can contribute. You breed community resilience: one person discovers that the local building company is giving away its scaffolding planks, so you get free scaffolding planks; someone else contributes gardening skills. We need neighbourhoods, and one of the best ways to construct them is around food, because growing food takes you to eating and eating takes you to cooking, sharing and swapping recipes. It's tangible in the way that growing flowers isn't."

A policeman told her that Capital Growth's £500 investment in a new green space was worth three bobbies on the beat. Boycott also believes that encouraging children to grow their own fruit and vegetables is the most effective weapon in promoting healthy eating. She used to be asked dismissively why anyone should spend public money on "growing carrots", but she thinks that the government is beginning to recognise the benefits. So far, the campaign has helped establish gardens in 300 schools in London, and she hopes to see one in every school in the country.

Yet the social benefits should not obscure the importance of locally sourced food. Some have estimated that a city of ten million people has to import at least 6,000 tonnes of food each day and, for a city as prosperous as London, the figure is far higher. Of the 6.9 million tonnes of food that Londoners consumed in 2000, some 81 per cent was imported into the UK. Ewen Cameron, the farmer who became the first head of the Countryside Agency, once remarked that Britain had become so dependent on oil to fuel its food supplies that we were only nine meals - or three days - away from anarchy.

The UK has a thriving farming sector - it exported £12bn of food and drink in 2007 - but it is far from self-sufficient in food production. We currently import 40 per cent of the total food we consume and the proportion is rising. Ben Reynolds, network director of Sustain, "the alliance for better food and farming", estimates that the UK imports 50 per cent of its vegetables and 80 per cent of its fruit.

“We grow apples in the UK but, even when they're in season, we still import them just because they're a few pence cheaper," Reynolds says. The short-term economic imperatives expose us to long-term risks. It would take decades to re-establish the orchards that we have "grubbed up" overnight and cheap imports will not always be so readily available.

That the cost of food has fallen steadily in the past 50 years, from 30 per cent of the average household budget to closer to 9 per cent today, is largely because oil has been cheap and plentiful. The global food chain relies on it for fertilisers, packaging and distribution. As the age of cheap oil comes to an end, so will the age of cheap food. "It's vital that we don't destroy our food culture," Reynolds says. "We're going to see food prices rise across the board and we're going to need farmers." He believes that the Capital Growth campaign might be one way of finding them. "If people get involved in food-growing through urban agriculture and decide to take it a step further, it might be a way of developing an interest in food production among a new generation."

London is not the only city in the UK to invest in urban agriculture. For the past few years, Middlesbrough has been running an urban farming and community growing project, which has led to the creation of almost 20 community allotments. Last year, the scheme involved two-thirds of the town's schools, dozens of community groups and approximately 4,000 people.

Catherine Boyle, of the Grown in Middlesborough project, says that the scheme produces enough food to ensure that everyone has something to take home at the end of the week and is giving some the chance to taste food they have never had before. Funding is now available to help people sell surplus produce and shift the project towards becoming self-sustaining.

Meanwhile, the London Food Board has said that it wants the capital to produce 5 per cent of the food that it consumes, but campaigners at Sustain suggest that the figure could be as high as 27 per cent.

There is certainly no shortage of space for food-growing, even in central London. A beekeeper called Mikey Tomkins, who has hives on the roof of the Royal Festival Hall, surveyed the space available in Elephant and Castle in 2005 and discovered that there was two-thirds more available than records suggest. "We have a weird blind spot for space in cities - people just don't see it," he said when I met him at St Mary's Secret Garden in east London, where he was involved in the launch of a campaign addressing the plight of bees.

As many food crops depend on bees for pollination, it follows that attempts to increase urban food production will require the protection of urban bees. The Capital Bee campaign announced the names of more than 75 competition winners who were later given training and equipment to establish 50 community hives and help tackle the decline in bee populations that extends beyond the city.

In the winter of 2009 and 2010, the UK lost a third of its bee colonies; the volume of managed European honey declined by 25 per cent between 1985 and 2005. The bee population in London is part of this trend. Last year, John Chapple, chairman of the London Beekeepers Association, said that his members had lost between a fifth and a quarter of their colonies over the winter. And yet Tomkins says that London could become a haven for bees. The city's mild climate and the limited use of agricultural pesticides and chemicals are its main advantages, though it also benefits from what he calls a "strangely unbureaucratic" approach to beekeeping and food-growing: "You do need to fulfil certain hygiene regulations but no one's checking on you. You can just jar your honey and sell it."

Tomkins, who is writing a PhD on community food gardening on inner-London housing estates, explains that people in other European countries operate under much stricter conditions than Londoners. The mayor of Copenhagen once told him that it was difficult to get permission to dig up public ground in his city, whereas in London, things tend to proceed by "word of mouth". "It's generally seen as a good thing and no one tries to stop you doing it," Tomkins says.

Historical precedents play their part. In 1800, Thomas Milne made a map of London's land use which showed how approximately one-fifth of the city was devoted to food-producing spaces, such as market gardens. The figure has fallen steadily ever since, though it rose again, out of necessity, during the First and Second World Wars. Cuba is a more recent example of what Tomkins calls a "crisis landscape". The country depended for much of its food on the Soviet Union. After the Soviet demise, the residents of Havana were forced to start growing their own on porches and balconies. In the early 1990s, the city was producing as much as 50 per cent of the fresh produce it consumed.

Tomkins acknowledges that it is much harder to make the case for urban agriculture in a time of relative plenty, but he believes that the opportunity to put in place new patterns should not be missed. "We have to establish that it's normal to grow your own food in cities," he says. "And if a crisis strikes, we'll be equipped to expand urban food production to a bigger area."

There is no doubt that small-scale, localised production can be remarkably efficient. A survey conducted in 1942 concluded that you could produce the same amount of food from 14 per cent of an acre of suburban housing as you could from an acre of farmland, for the simple reason that people work smaller plots, such as gardens, much more effectively.

The lesson has not been forgotten entirely. In 2008, a chef called Kate de Syllas set up a herb garden on the Hackney estate where she lives, hoping to show that small, urban spaces can grow good produce and deliver it to "market" efficiently. Wenlock Herb Garden occupies
an unpromising, quarter-of-an-acre plot beside a basketball court. There is a row of garages on the far side of the road and the frame of a half-built office building on Old Street rises above the surrounding blocks of the estate. Yet the woodchip paths of the garden enclose a rich array of herbs and plants on raised wooden beds - familiar varieties such as rosemary, thyme and sage are complemented by purslane, white borage, red dandelion, fat hen, bronze fennel and other more exotic plants, such as the Japanese wineberries cultivated by the Australian gardener.

De Syllas uses some of the produce at the restaurant on Kingsland Road where she works; most of the rest is sold to other chefs. The garden lost one of its customers last year when a restaurant called Konstam, which aimed to source all of its food within the M25, went bust, but it has a prestigious customer in Fergus Henderson of St John Hotel, which recently opened in Leicester Square. It uses Wenlock Herb Garden's salad burnet - a herb that tastes of cucumber - in one of its starters.

Capital Growth's "adopt a plot" scheme attempts to encourage other restaurants to strike up similar relationships with their local food-growing spaces. "At the very least, it starts a conversation about what it is possible to grow in central London," de Syllas says. "Maybe we can stop some of the rocket coming from Italy. Obviously, it's a drop in the ocean, but more and more people are seeing that it can be done."

Wenlock Herb Garden's set-up costs were modest - it received two grants of £2,000 from Capital Growth and the tenant management company on the estate. In its first full season, it produced 300 kilograms of food.

De Syllas believes it could be run at a profit if it were twice as big. Smyth of Farm:shop shares her ambition and wants to make his urban agriculture scheme a viable business. "You can't do farming as a pop-up [venture]," he says. "We've seen projects in Paris where people set up veg beds and they're gone the next day. That didn't seem very meaningful to us."

For the time being, Farm:shop is raising funds by renting out office space on the top floor of its building, but hopes to find new ways of creating "sustainable livelihoods from food". It is exploring the possibility of growing saffron on a housing estate in Essex and is replicating de Syllas's strategy of growing unusual crops for London's diverse and sophisticated markets. "Local isn't local any more in a globalised city," Smyth says. "Since we don't want to give up the rich diet we now enjoy, technologies like aquaponics can fill a gap and help us to grow things such as pak choi, which would otherwise be imported."

Above all, Farm:shop hopes to exploit the obvious advantage that urban food-growers possess - proximity to the market. "We're trying to find a simpler way of getting food on to people's plates than the abominably complicated system we've got now. You should be able to pick it and sell it where you pick it."

Edward Platt is a contributing writer for the New Statesman