Sellers using fake identities online to conceal illegal trade of puppies
A BBC Scotland investigation to be broadcast later will expose a multi-million pound world of dog trading.
The investigation discovered traders hiding behind fake identities online to profit from the illegal sales of puppies.
They found one such dealer, Liz Baird, who had previously been prosecuted and banned, advertising puppies for sale online under a number of false names.
The cameras have been following the supply chain from source to sale.
In the documentary, Michaela Harvey says Ms Baird sold her a Jack Russell-Pug crossbreed puppy for £395 in an Ayrshire car park.
The dog, named Bane, fell ill and died three days later.
'Worst sound ever'
She said: "He was just lying and he wasn't moving at all, but he was still breathing. He let out a kind of scream.
"It was the worst sound I've ever heard in my life. He was sick everywhere and he just died."
Ms Baird is said to be a notorious dealer within the puppy industry.
Reporter Samantha Poling also investigated Bernadette and Brian O'Neill, from Hamilton.
She found the couple took out dozens of adverts to sell puppies in just a few months and used different names, which concealed the scale of the operation.
BBC journalists posed as potential buyers of two puppies and secretly filmed the meeting at the couple's home.
Bernadette O'Neill said the dogs had been vaccinated, fully vet-checked and were supplied by a breeder in West Kilbride. However, no licensed breeders could be found in the town.
Secret filming of Mr O'Neill revealed he was making buying trips to the Republic of Ireland and illegally bringing back hundreds of dogs to Scotland, without pet passports or rabies injections.
Both Ms Baird and the O'Neills were asked to participate in the programme but both declined to do so.
BBC Scotland Investigates also travelled to Northern Ireland, where the dog breeding industry is worth £160m, and filmed at night inside Furnish Kennels, run by brothers David and Jonathan Hamilton.
During filming at the kennels, the BBC found hundreds of breeding bitches in battery-farmed conditions.
In a statement through his solicitor, David Hamilton said his premises "are maintained and run in accordance with all relevant legislation and regulations in an entirely open and transparent manner".
He added: "All dogs are under the supervision of a nominated veterinary surgeon."
The council responsible for the inspections said the kennels were inspected four weeks ago. They said the premises complied with legislation.
The Dog Factory will be broadcast at 21:00 on Wednesday 15 April on BBC One Scotland, and BBC One Northern Ireland.