Teaser is a 10-stone Staffordshire bull terrier cross-breed, flanked like a horse and with a head the size of a rugby ball. It lives in a small flat in Somers Town, central London, and local kids on the estate often knock and ask to walk it, amazed that the 22-year-old owner, CJ, can make it sit and give paw. Many of the kids have their own dogs – staffie crosses, rottweilers, and illegal pit bulls – and the numbers are multiplying as they are bred with other dogs on the estate.

"It's a status thing – one or two people get them and then everyone's got one," says CJ. "Kids think I've got a pit and I'm a hard man, but they're the ones running around estates being pulled on leads. [The dogs] keep spreading because they're so easy to get. Just go on to the internet and type in 'pit bull puppies'. It's not surprising everyone I know has a dog."

Yet a growing number of dog owners are irresponsible. In London, the number of dogs seized by the police under the Dangerous Dogs Act (DDA) soared from 263 in 2006-07 to 719 in 2008-09. So far this year, 1,000 dogs have been confiscated – a fourfold increase in three years.

The act allows dogs to be seized because they are illegal, dangerously out of control, or if they are used to threaten or intimidate someone. So legal breeds, such as Staffordshires, can be seized under the act. Pit bull-type terriers, Japanese tosas, and the dogo Argentino and fila Brasileiro mastiffs are all illegal breeds, but many owners get around the rules by mixing illegal breeds with Staffordshires and calling them crosses.

The increase in seizures in London has been driven by a crackdown and the opening of a Metropolitan police Status Dogs Unit (SDU) last March, but national data suggests there has also been an increase in violent dogs on the street. According to the Ministry of Justice, the number of dog owners prosecuted for causing injury rose by 50% between 2003 and 2007. The RSPCA says that calls about dog fighting have increased massively over the last few years, with two-thirds of complaints now directly connected to young people using dogs as "weapons" in streets and parks.

A series of high-profile attacks have focused public attention on this issue. Last February, a baby died in south Wales after being attacked by a Staffordshire bull terrier and a Jack Russell. In November, four-year-old John Paul Massey was mauled to death by a pit bull in Liverpool. And just before Christmas, a guide dog was viciously attacked by another dog, whose young owner was walking it unleashed by in north London. CJ says that walking without a lead is too often used as a sign of power and control on the street. "I'm always paranoid that some dog off a lead is going to attack Teaser," he says.

David Grant, director of the RSPCA's Harmondsworth hospital in north London, picks up the pieces of irresponsible dog ownership every day. His hospital is full of emaciated staffies that have been abandoned – often in the hospital's reception – by young men who don't want to give their names or pay veterinary bills. Many of the dogs are sick, as their owners haven't had the money or the knowledge to apply for vaccinations. In the worst cases, dogs have been abused by irritated owners or been forced to fight.

In 40 years of practice, Grant, whose veterinary operations were seen by millions on the BBC's Animal Hospital television series, says things have never been so bad, and he has started documenting the worst cases. His computer now hold hundreds of images of dogs that have been shot, stabbed or burnt.

"A typical problem owner will be from an inner-city estate, unemployed, without any educational achievements," he says. "Young males predominate, although the fighters often register the dog in a girlfriend's name." Names such as ­Terror, Chaos, Killer, Ice and Asbo often tell a dog's story, says Grant, as does evidence of ­harnesses – accessories often used to glamorise dogs before gang fights.

But Grant is keen not to sensationalise or oversimplify. He wants to distinguish between "fashion dogs", which are simply part of a craze, and "status dogs", which are bred for offence and defence. "Fashion dogs tend to be staffie crosses that are naturally good-natured, turning nasty only when they suffer abuse, or neglect when their owners get bored," he says. ­"Status dogs, on the other hand, are bred to intimidate. At the worst level, gangs will use them for mascots, muggings, safeguarding territory, and fighting enemies and other dogs."

Grant, along with dog wardens and police officers, has repeatedly drawn attention to the parallels between dogs and knives. Both are carried by young ­people in areas where crime is high, often for defence. In some cases, even parents have been known to encourage their children – particularly girls – to walk with dogs as a means of protection. However, in contrast to ­possession of knives, a young person will not face five years' imprisonment for having a dog.

But if fear and fashion are multiplying dog numbers, so is the potential money that unemployed young people can make from "dog dealing. Staffordshire puppies and their crosses can sell for £400-£500 each, and with an average litter size of eight and a bitch able to produce two litters a year, an owner can earn up to £8,000 annually from a single dog.

From his council flat in north London, Dion, 24, supplements his living by dealing dogs. He's got a few scars from violent dogs he's owned in the past, but now he's got just one, a mixture of a staffie-pit cross and a presa Canario. "My dog has had her first litter and two generations of grandchildren," he says. "I wanted to keep the bloodline going. The money's not the ­priority though – I'll sell them for half of the £350 I could get when I know they're going to a good home."

Shredded forearm

Security is another reason Dion hangs on to his dog. He says: "My dog attacked my stepdad once – for good reason. He was a bit of an alcoholic, and when my sister got scared of him once she screamed and the dog went straight for him and shredded his forearm before he could hurt her. It's another reason to feel safer."

According to Sergeant Ian McParland, chief officer at the SDU, simply banning more dogs under the DDA is not the answer. The problem is not genetics, but upbringing, he says. Most dogs can become aggressive or peaceful, depending on how they're raised, he says.

"You could go on banning breeds until the cows come home and it won't make a difference," McParland says. "We're almost fortunate that the status dog of choice, the pit bull terrier, is illegal. I don't know what we'd do if Akitas, German shepherds and rottweilers started becoming fashionable [as status dogs]. Akitas were used by Samurai warriors."

He points out the threat posed by selective breeding. "Breeds are getting more dangerous," he says. "If you've got a nasty dog and your mate's got a nasty bitch, they're the ones you're going to breed."

So far, local authorities' response to the problem has been mixed. Last month, ­the London borough of Harrow proposed ­vetting and chipping all dogs owned by people waiting for social housing. Anyone with a dog that it suspected of being used for fighting would be refused a tenancy, though deputy council leader, Susan Hall, insisted that this would be a last resort. "We will work with the RSPCA and police to make sure that people in council properties don't keep dogs that are a menace to others," she said.

In south London, Wandsworth council is already piloting a programme that will see residents threatened with eviction if they fail to keep their dogs responsibly. Other London councils employ specialist dog wardens who are on call to answer residents' problems; others simply slide dog issues into the files of the resident environment officer.

In Liverpool, following the death of John Paul Massey, councillors voted for an amnesty on illegal dogs, free micro-­chipping, and tougher rules on leads and muzzling. They are also investigating a dog registration scheme.

Root causes

According to Grant, it will take more than punitive measures to curtail irresponsible dog ownership. "A lot of the time I feel as sorry for the owners as the dogs," he says. "These young men have been on a conveyor belt of social deprivation since the day they were born, and we're at the end of it, trying to pick up the pieces. They've had no upbringing, and they've got no educational qualifications and no prospects. Society has let them down. We need to address the root causes if we're going to solve these problems."

Back on estates in north London, young people are talking about breeds, bloods and lineages. Videos of dogfights recorded on mobiles are changing hands, and an old cage for dog fighting sits in a garage.

Talking to CJ, it is clear that owning dogs is a way of life. But it is one that he wants out of. "I want to take my life out of the city so that she's safer," he says, play-fighting with Teaser. "There are so many kids in London with dogs these days, and they all want beef [trouble]. At the end of the day, I love dogs more than people. I'd move to the country to keep her safe."

Some names have been changed.