It's 11pm on a muggy Friday night in Docklands. And while the rest of the city is out carousing, Elle Dunn and her boyfriend, Rez, have just vaulted an 8ft high fence, scudded through a thicket of thorns and now find themselves crouched behind a bush, engaged in a high-octane game of cat-and-mouse with a security guard.

Ahead of them is the hulking monolith of Millennium Mills, disused since the 1980s and a decaying industrial anachronism standing defiant and alone in the surrounding subtopia. Their mission? To infiltrate the building, described by others who have visited it as a “death trap”.

It's eerily quiet. Following close behind, my hair is damp with nerve-induced sweat. And there's a two-inch gash in my arm from a rusty nail. But there's also a palpable thrill, especially given the fact that what Elle and Rez are about to do is illegal. Suddenly, the guard's SUV engine roars into action. Elle darts into bracken, hurtling through razor wire and crushing a DANGER! KEEP OUT sign underfoot. “Come on,” she says, pointing to an insurmountable-looking fence. “We're going inside.”

A gamine 24-year old with hair dyed dark red to evade security (it's less likely to pick up on the cameras), Elle loves derelict places and is fearless and articulate. She's one of the growing band of urban explorers — a loosely connected cabal of thrill-seekers, photographers and building enthusiasts who assume aliases such as Rooftop Vermin and spend their spare time infiltrating the unseen, off-limits nooks and crannies of London's past. What they do is illegal and can't be condoned but they're in it for the thrill and not to damage, burgle or endanger. Their motto is: take only photographs, leave only footprints.

At this moment, they could be abseiling the chimneys of Battersea Power Station or wandering through the capital's many abandoned factories, asylums, swimming baths and nuclear bunkers.

Somebody could be kayaking down London's labyrinthine network of sewers (“Russian Roulette” according to one urban explorer). Or, they could be snaking their way into Bow Street Magistrates Court or the Big Brother house, merely to have a nosey around.

Although this is trespass, the aims of urban explorers are, for the most part, benign. Many (such as Elle) are photographers who see beauty in urban decay, posting pictures online. Others view themselves as “industrial archaeologists”, performing a vital role in recording the history of such places. Meanwhile, some just do it for the sheer excitement of scaling or tunnelling into a building without being caught. At the most extreme end of the spectrum, there are groups such as the Craniacs, who take their lives in their hands by ­clambering up a 245ft crane in Docklands, as they did last year.

The movement has its roots in 1950s America, when bored students at the Massachusetts Institute of Technology led tunnel-hacking excursions into the subterranean passageways around their campus. But the term urban exploration wasn't coined until 1996 when Canadian Jeff Chapman (or “Ninjalicious”) published a fanzine called Infiltration, which gave advice on such things as navigating storm drains and circumventing hotel staff in order to use their five-star health clubs.

Over the past decade, the activity has swelled globally thanks to the internet, with branches everywhere from San Francisco to Paris, where urban explorers hold impromptu parties in the catacombs beneath the city. The UK's main online forum, 28dayslater.co.uk (named after the post-apocalyptic Danny Boyle film), has doubled its membership over the past two years to nearly 10,000.

Standing opposite the ExCeL Centre at Royal Victoria Docks, Millennium Mills was built in 1933 and in its heyday was Europe's largest flour mill. Derelict since the mid-1980s, it featured as a backdrop in Jean-Michel Jarre's Docklands concert later that decade and is now occasionally used as a filming location for programmes such as Ashes to Ashes. Although plans are afoot to turn it into luxury flats as part of the Silvertown Quays development, Rez warns that the structure is “one of the most dangerous urban exploration sites out there”.

Having successfully avoided impalement on the fence's viper-spikes, Rez leads the way to a small hole in the wall of the building. Crawling through this L-shaped aperture on hands and knees, they emerge in a flurry of dust onto a floor covered in broken glass, wrought-iron girders and mildewy wooden planks. “This musty smell is like heaven for explorers,” whispers Elle as they ascend the stairs. “This is such a sexy building …”

Like a booby-trapped House of Horrors, danger awaits their every step in Millennium Mills. The rotten floors are comparable to thick slices of Emmenthal, riddled with pigeon faeces and yawning holes (where machinery has been removed) that drop eight or nine storeys in some places. As Rez says: “Enter without a torch and you won't last long.”

Creeping along the sides of the building, ducking whenever the light of the SUV goes past, they pass sawn-off staircases and rusty helter-skelter slides until they arrive at possibly the most hazardous part of the building: the five-foot “leap” from one mill to the next. In order to do this, they climb through a shattered glass window frame, stand on a ledge three storeys up and walk down two planks positioned at a 45-degree angle. Elle and Rez scramble across, monkey-limbed and with all the aplomb of Man on Wire's Philippe Petit.

It was visiting ruins with her mother while growing up in France that gave Elle her first taste of the desolate beauty of derelict places. Five years ago, while studying for an arts degree, she started to take the hobby more seriously. “I felt the need to document these buildings by taking photos,” she says. Now, visitors to her Contamination Zone (www.contaminationzone.com) website can see HR Giger-esque shots of places ranging from Nazi sanatoriums to children's hospitals, where the antiquated wheelchairs are covered in moss. It's all testament to how spectrally photogenic the places are, as well as the hours spent trawling through Google Earth looking for new places to explore. She also has the scars to show for it.

While exploring a piano factory alone a few years ago, Elle fell, splitting her back open on a scrap-metal heap, which resulted in 27 stitches. Luckily she had mobile reception and managed to call an ambulance. “I cried more when I broke my camera,” she laughs.

Elle's escapades have also led to her being questioned by Scotland Yard on terrorist charges after emerging unwittingly from a sewer exploration near Buckingham Palace and stumbling across an ex-patient on a solo expedition through tunnels underneath West Park asylum. Having met Rez at a party inside Battersea Power Station a year ago, Elle and he are now the Bonnie and Clyde of urban exploration. “We've never been on dates to restaurants or the cinema,” says Rez. Instead, they spend their weekends hanging out in some of the most exclusive and romantic spaces possible: among the ghosts and atrophying grandeur of forgotten, forbidden London.

It would be easy to write urban explorers off as irresponsible adrenaline junkies with a kamikaze-like attitude towards self-destruction. But they always set out in groups and each expedition is planned with a near-military precision, the result of hours of research. Even the Royal Society for the Prevention of Accidents concedes that: “Although we don't endorse it, these people are very skilled and know what they're doing.”

The dangers nevertheless are manifold and abandoned structures can contain hazards such as asbestos, poisonous gases, exposed electrical wires, security dogs, unsound floors and potentially violent metal thieves. But in a society so buffeted by health and safety regulations, urban exploration represents one of the last opportunities for real adventure.

There is also the legal aspect. Although trespassing is a relatively minor civil offence, urban explorers avoid damaging the properties as it could lead to being charged with the more serious criminal charge of breaking and entering. “You'll often find that a lot of urban explorers have ingenious methods of getting into places, such as finding tunnel systems underneath fences or ventilation ducts,” says fellow explorer Simon Cornwell, who specialises in asylum visits.

Another taboo is snaffling keepsakes, whatever their value on eBay. “If you pick up anything, whether it's decaying papers or a rank pair of shoes, it makes it easier for the police to charge you with burglary,” says Cornwell.

It's now 1.30am and having navigated their way out of Millennium Mills, Rez takes Elle to a CCTV-festooned pontoon moored on a stretch of water opposite. Inside a cabin, there's a makeshift bar with dusty bottles of wine, some mouldy paper plates and an out-of-tune piano — like so many neglected structures, a snapshot of the day the place was last frequented. Outside, the neon reflections of Docklands' skyscrapers illumine the water.

There's an unearthly calm and it dawns on you that this illicit serenity is a quintessential part of what urban exploration is about — being privy to views, thrills and experiences few Londoners sample. Sitting on the wooden decking, Elle muses on her ultimate dream. “One day, I'd love to be the Queen of my own derelict palace,” she says. Having turned the crumbling buildings of Britain into their own nocturnal playground, Elle and the rest of the urban explorers are halfway there already. Long may they reign.