A few hundred metres from Clapham North tube station stands a padlocked gate. Behind the gate is a dark, damp entrance to a spiral staircase leading 33 metres underground. A series of tunnels built as a second world war bomb shelter large enough to fit 8,000 people have remained virtually unused. Until now. At the end of one tunnel comes a pinkish-purple glow from behind white plastic sheeting. The Breaking Bad comparison is obvious. But the produce being grown using hydroponics and LED lights isn't illegal. It's salad. Salad, the taste of which is liked by no less than chef Michel Roux Jnr.

Everyone who comes down into the tunnel is initially very sceptical, laughs Steven Dring, who along with his friend and business partner Richard Ballard is the man behind Zero Carbon Food. "But then they say, 'OK, let's make this work.'"

The project has been in development for the past two years, and has attracted the interest of the mayor of London, naming Dring one of this year's 'London Leaders'. It is, as of today, looking for finance via the crowdfunding website Crowdcube. Only using a small part of one tunnel so far, the space they have leased from Transport for London (TfL) gives them a potential for 2.5 hectares of growing space. The produce including pea shoots, rocket, red lion mustard, radish, tatsoi, pak choi and miniature broccoli will be branded as Growing Underground and aimed at the retail market and high end restaurants.

Roux, a longtime Clapham resident, needed little convincing to come and visit. "There's a growing demand for sustainably, locally grown produce in London," he says . "I thought they were absolutely crazy but when I visited the tunnels and sampled the produce they are already growing down there I was blown away. The market for this produce is huge."

However, what possible sustainability credentials can food grown under artificial, energy consuming lighting have compared to soil and sunlight? "Open field and greenhouse farmers are affected by low light, weather, pests, all of those issues", says Dring. "Between 2009-2012 food inflation ran at about 32%. That's because of issues with crop production and failed crops … down here we have no pests and a consistent temperature of 16C. Once we've put all the LED lights in they give off a little heat that will take us up to about 20C, perfect growing temperature."

If additional heat is needed, there is an obvious local source – diverting thermal flows from the Northern Line. Electricity is currently bought from a renewable energy supplier, but Dring has committed to generating onsite renewables through (above ground) wind and solar, with the intention of being a net exporter to the grid. A costly sump system currently pumps huge amounts of water out of the tunnels, water that the team have had tested and is suitable for growing. Dehumidifiers will also take water from the air expired by the plants, to be recycled. They are even looking into recycled carpet as a sustainable substrate.

"As much as hydroponics sounds technical, it's actually very low-tech", says Dring. "It's flooding a bench full of seeds, like growing watercress as a child. The water then ebbs back to the tanks before flooding again hours later, and so on. It's not at all energy demanding … The constant temperature down here is a reason not to do it under glass, on the surface."

Conventional farming may seem natural, says Dring, but among commercial operations use greenhouses or polytunnels that typically require artificial heating, many also use LEDs in addition to sunlight. Transportation adds a significant cost and carbon footprint to food taken from rural farms to large population hubs such as London. Zero Carbon Foods is barely three miles from Covent Garden market, the wholesaler those same farm lorries are trying to reach. Growing Underground can be there within two to eight hours of being picked, giving it a longer shelf life, transported by electric vehicle.

As well as Roux, Dring and Ballard have Neil Sanderson, the MD of salad giant Florette as a non-executive director. It's a sign that the industry is taking this seriously.

"Eventually we'd also like to go beyond underground to grow vertically – use the tiny footprint of a high rise and convert it into a farm," says Dring. "Urban farming will be the farming of the future." There have been stories of rival projects, such as mushroom growing in rail tunnels below Oxford Street, although "the ones we've heard of are all conceptual", says Dring. "We are way past the concept stage. But if other people are going to do this then fine – we want this to spread.

"But let's not get ahead ourselves", he adds, swiftly. "It took us two years to get through the testing phase, now we need to raise the funds to build it." The hope is to have a goods lift fitted in late summer, with full kit-out to follow shortly after. Much of the produce can then crop within one week.

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