I have always wanted a dog. As a child, I pored (pawed?) over the pet classifieds in the local newspaper, looking for the perfect puppy (ideally the reincarnation of Timmy from the Famous Five books). I badgered my animal-phobic mother for a dog on a daily basis, just in case she happened to change her mind. She never did - though I eventually wore her down enough to allow a hamster.

What I didn't realise was those border collie pups I lusted after in the paper - along with the labradors, cocker spaniels and hundreds of other breeds - were probably raised in appalling conditions. Reputable pedigree dog breeders rarely advertise litters in newspapers or on the internet. Chances are, those dogs came from puppy farms.

Puppy farms - large-scale commercial breeding establishments - hit the headlines whenever an undercover TV crew exposes a particularly disgraceful example of animal cruelty. There is a brief wave of outrage, followed by relief all round that the perpetrators have been brought to justice.

That relief, however, is misplaced: the exposés are only the tip of the iceberg. Animal charities report that puppy farms are dotted across the UK, with a particular concentration in Wales. The RSPCA estimates that 50,000 farmed puppies are imported (or 'trafficked') into the UK every year from Ireland, where breeding legislation is practically non-existent. A significant proportion of the UK's estimated eight million dogs started their lives in a puppy farm.

In a campaign launched last month, the Dogs Trust has renamed these large-scale breeding operations 'battery farms for dogs'. This, believes the charity, gives a more accurate impression of what conditions are like for the dogs that live - and often die - there. The term puppy farm conjures up a rather bucolic image of Andrex puppies gambolling about in fields of clover. The reality, says the Dogs Trust CEO Clarissa Baldwin, is rather different. "These places are disgusting: dark, dirty, dingy and smelly," she said.

Bitches are isolated and given the minimum food and water needed to keep them alive and breeding. They have little veterinary care and no exercise, stimulation or affection. There are bred from continuously until they are worn out, and when they are of no more use they are killed. The staffing levels are completely inadequate: campaign group Puppy Love cites one farm where a single man oversees 150 dogs.

The Dogs Trust's latest campaign poster. Photograph: PR

Cath Gillie, an assistant field director at the Dogs Trust, has witnessed the conditions at puppy farms first-hand. She recalls the sheer scale - more than 100 dogs crowded into stalls; the smell - an overpowering blast of ammonia; and the noise - continual barking for attention.

"One dog tried to jump out of its stall and into my arms," she said. "Others were very nervous and cowered at the back. They had no toys, no bedding, just bare concrete."

It is hard to overestimate the health impact on puppies starting life in such conditions. Common problems include canine parvovirus, worms, hip dysplasia, dislocated kneecaps, and congenital heart problems. Clare Marklen learned the health risks the hard way. She bought a miniature Jack Russell she had seen advertised online. As soon as she got the puppy home it became ill, passing blood and diarrhoea, and had to be taken to the vet. The following day, she found the puppy dead in its basket.

"I was so angry," she said. "Not about the money [£295 for the puppy plus vets fees] - but about the puppy's life."

Marklen, like many owners, initially had no idea she was purchasing a commercially bred puppy. The ad was misleading. The puppy she was given didn't resemble the ones in the picture, the breeders wouldn't let her see the dog's mother, and it looked too young to be sold. But she couldn't bear to leave a tiny puppy in a dirty house, where the owners were churning out dogs on the ground floor and cats upstairs.

This is a mistake that many people make, say campaigners. However well-meaning, 'saving' a puppy simply fuels the dog farming trade and condemns more bitches to lives that aren't worth living. It is far better to report any unscrupulous breeders to local authorities.

Physical health problems are only one side of the coin. Farmed dogs' mental health is equally likely to be damaged during the crucial early weeks of development. Starving a dog of animal and human contact prevents it being socialised - learning how to relate to its owners and to other dogs. While a young puppy may still learn social skills in its new home, not all breeds respond in the same way, and for some it can be too late. Dogs may become aggressive, fail to bond with their owner, or, conversely, become overly attached. Breeding bitches may reject their puppies, or even attack them.

"These aren't farm animals, bred for food. They are pets, bred for companionship", says Gillie. It is a cruel irony that humans selectively bred those varieties of dog that thrive on human company - and are now depriving them of that very need. It is unsurprising that such dogs display behavioural problems. As Gillie explains, they never "learn how to be a dog."

Puppies before they are taken from their mother. Photograph: Puppy Love

So what can be done about the trade? While experts are divided on whether the current animal welfare legislation (see below) is adequate, all agree that a more pressing problem is enforcement. Local authority inspectors lack specialist knowledge; they are understaffed and don't have the resources for spot checks; and it is too easy for breeders to use loopholes in the law or falsify papers.

Various initiatives are under way. The Bateson report into dog breeding, published earlier this month, recommends establishing a statutory code of practice under the Animal Welfare Act 2006. Bateson also recommends compulsory microchipping before the point of sale, so dogs can always be traced to their breeders. The British Veterinary Association (BVA) is in the process of drawing up a 'puppy contract', designed to protect both vendor and purchaser. The Pet Advertising Advisory Group (PAAG) investigates cases of problem dogs that are bought from classified or online ads.

But ultimately, all those attempting to stamp out the trade believe that educating consumers is the only way to succeed. "The law is an important part of it, but the consumer makes or breaks these puppy farms," says Baldwin. Harvey Locke, president-elect of the BVA, agrees. "Legislation has to be regarded as a backstop", he said. "The BVA feels the most important thing is education."

The market in dogs, like any other, obeys the law of supply and demand. "While there's a demand for cheap puppies - like cheap eggs - there will be people willing to meet that demand," says Gillie. She believes that if consumers knew their dog and its mother had been raised in good conditions, they would be prepared to pay more - just as they pay more for free-range eggs.

Puppy Love is less optimistic. Public education is a lengthy process, and the messages need to be continually reinforced. "These dogs need help now; they're dying now", said a spokeswoman. "They've waited long enough." It may seem counter-intuitive, but the best way to help mistreated dogs is to buy a happy, healthy dog from an approved breeder. Only then will the business of farming dogs die out.

I stopped trawling the classifieds years ago, and as a flat dweller I'm still not ready to fulfil my Famous Five fantasies. But when I am, I'll be calling the Kennel Club or visiting a dog shelter. Farms are for livestock, not pets.

The dos and don'ts of buying a dog

Do get advice from your local vet before buying. Vets are more than happy to recommend breeders and would much prefer to help you at this stage than when you have a sick puppy.

Don't  ever   buy from a pet shop. The vast majority are supplied by puppy farms.

Don't buy a pedigree dog at a bargain price after seeing it advertised in a newspaper or online. It is not genuine. Accept that a dog is expensive to buy and to care for (at least £10,000 over its lifetime); if you can't afford it, don't buy it. Buy from Kennel Club Accredited Breeders or breed rescue organisations.

Do consider classified ads if you are happy with a mixed breed dog, but agree you are buying it on condition that it passes a medical - and get this in writing. Take it to the vets within 48 hours. Or, even better, buy from an animal rescue centre.

Do go to the breeder's premises to view the puppy, so you can see the conditions first-hand. Breeders may offer to meet you at a convenient halfway point - never accept.

Don't ever buy a puppy if you can't see it interacting normally with its mother. Farmed dogs are taken away from their mothers early on, but a replacement dog may be placed in the room while you are viewing it.

Do check the paperwork. All breeders should be able to provide you with medical records. Pedigree breeders should have Kennel Club registration papers (must be Kennel Club not anything else which is 'made up'), and certain breeds should have parents' hereditary disease screening certificates.

Don't buy a dog on a whim, or as a gift. Research the breeds that will suit your lifestyle. Bear in mind that a dog needs one to two hours of exercise a day and can live for up to 15 years.