Undercover footage from a BBC Panorama investigation into puppy farms reveals the "ruthless world" of the lucrative industry.
Britain’s Puppy Dealers Exposed, which airs on Monday night, will see reporter Samantha Poling tracking the supply chain of Britain's favourite pet.
Recordings from inside Irish puppy mills show scores of breeding dogs being kept in ramshackle cages, with water supplied to them by pipes that are normally seen in battery pig farming.
Marc Abraham, vet and founder of Pup Aid, which campaigns to end puppy farming, told Panorama: "This is an industry built on lack of transparency, deceit, cruelty and animal suffering."
The multi-million pound puppy breeding business has come under increased scrutiny in recent months.
The RSPCA believes that huge numbers of puppies are being trafficked into the country from other EU countries to meet the high demand for pedigree and designer cross-breed puppies in the UK.
A report released by Battersea Dogs and Cats Home in October revealed how the unregulated breeding industry was putting dogs at risk.
The London-based rehoming centre's report showed that less than 12% of puppies in the UK are bred by licensed breeders, meaning that dogs could be sold from unsuitable premises, long before they are ready to leave their mothers.
BBC reporter Poling spent six months investigating the darker side of the puppy trade.
Secret filming at Raymond Cullivan’s puppy farm in County Cavan, Republic of Ireland, shows dogs giving birth in confined spaces, unable to move around freely.
These boxes, which offer little or no ventilation or daylight provides no space away from the pups, are illegal in Ireland.
Yet despite such breaches of animal welfare legislation, many of these pups are bound for the UK market.
Eric Hale's beagles are Kennel Club registered and have qualified for Crufts in the past. Poling describes him as "one of Britain’s most prolific dog dealers".
Every Tuesday he loads his van with crates of pups, travels by boat to Liverpool and drives around the country dropping dogs off to the next link in the supply chain.
From large scale sellers, to country layby dealers, Hale delivers to all, Poling says.
She adds: "Hale is in the big league. We discovered he was licensed for 120 breeding bitches.
"With a puppy farm of that size, it’s harder to control disease and give each dog the human attention it needs if it’s to be a happy family pet."
The reporter films in secret at Hale's puppy farm at 2am. It is minus six degrees and the team records using night vision cameras.
Poling says that the behaviour of the dogs is "disturbing to watch".
Many of the dogs have little or no bedding and, after watching the farm for several days saw, she no sign of the dogs being routinely taken out of the barns.
Hale said in a statement that his kennels met “all the requirements for a breeding establishment”.
Animal legislation experts told the BBC programme the facilities did not provide adequate barriers to prevent disease and that the dogs were "basically in jail" and "seriously deprived".
Poling said that most of the puppies end up being sold online by dealers who will often try to hide where the dogs have come from, with adverts implying the dogs have been born and brought up in a family home.
Abraham said: "In 2016, we should be better than this. We shouldn’t be farming dogs on a mass scale.
"They feel pain, they feel suffering, they feel fear.
"The only people benefiting are the irresponsible breeders and the dog dealers."
Britain’s Puppy Dealers Exposed will air on BBC One at 7:30pm.