A BBC investigation has uncovered a network of criminal gangs who supply illegal pit bull terriers for fighting.
Pit bulls are banned under the Dangerous Dogs Act 1991
An undercover Panorama team found that the dogs, which are banned in the UK, are brought into the country using false documents.
The dogs are smuggled across Europe for organised fighting and Northern Ireland is the epicentre of the network.
Pit bulls have been banned in the UK since 1991 and dog fighting has been illegal since the 19th Century.
The gangs breed, train and fight the unregistered pit bulls - the type of dog which killed five-year old Ellie Lawrenson in St Helens in December - and Panorama spent 17 months infiltrating the illegal world of dog fighting.
The undercover team managed to gain access to one of the biggest gangs - the Northern Ireland-based Farmers Boys.
Stephen Philpott, from the Ulster Society for the Protection of Animals (USPCA), told the Panorama team the Farmers Boys were talked about with the "utmost respect".
"The Farmers Boys are absolutely huge, they are massive, they are the Manchester United of the dog fighting world," he said.
"Over the last 25 years they have established trading partners in inner city Britain and they are now selling their dogs to those people in Glasgow, Edinburgh, Manchester, Liverpool, London."
Ellie Lawrenson was killed by a pit bull terrier last year
BBC reporter Mandy McAuley, who was part of the undercover team, said the gangs took advantage of the fact Northern Ireland has a "soft" border with the Republic of Ireland, where it is not illegal to own a pit bull.
"They can bring in dogs from other countries to Dublin, by plane, then drive them up into Northern Ireland," she said.
"Then they can either keep the dogs for their own breeding or fights, or they put them on a ferry over to Britain. We did this ourselves and saw how easy it was."
The team went to Finland and bought a pit bull from a breeder, who provided fake documents identifying the dog as a boxer-Labrador cross.
They also witnessed a fight in Finland which ended in the death of one of the dogs, and where a badly injured dog was wired to the mains and electrocuted.
In addition to the pit bull trade being about money, with big bets laid on fights, Ms McAuley also said some people were involved for the love of the "sport".
She said gangsters, as well as those involved in dog fighting, were buying the animals, which have become a status symbol in the UK's inner cities.
"Even before Ellie Lawrenson was mauled to death by her uncle's pit bull on New Year's Eve, people in Liverpool knew what the dogs could do," she said.
Panorama will be broadcast on BBC One on Thursday 30 August at 2100 BST.