Gangs of animal traffickers could be making more than £100 million by selling puppies brought into Britain illegally.
Disease-ridden dogs which were bred in unhealthy conditions are being trafficked into Britain on fake papers, according to the Daily Mirror.
Chief inspector Ian Briggs, from the RSPCA's Special Operations Unit, said there could be up to 100 of these criminal gangs operating "like drug cartels".
Mr Briggs told the Daily Mirror: "These gangs are highly sophisticated and they have huge earning potential. The level of fraud is staggering.
"The gangs are faking dog passports, forging documents and doctoring their routes across Europe to make them seem like they are legitimate businesses.
"We estimate the illegal industry is worth in excess of £100 million."
He said many animals are ill before they arrive and others could be carrying diseases such as rabies which would have a huge public heath risk.
Mr Briggs said: "The gangs pretend to be legitimate breeders but they are hiding the fact they are turning over hundreds of dogs as quickly as possible with no regard for the animals' welfare.
"The new owners are saddled with huge vet bills or the dog dies within a few weeks."
It is claimed that the animals have been trafficked across Ireland and eastern Europe .
A designer cockapoo or French bulldog bred at a puppy farm in Ireland or Poland might "cost 100 euros (£75) but could be sold for £1,500 over here", Mr Briggs said.
The RSPCA said that in the last three years it had saved more than 1,000 dogs from being trafficked, kept on puppy farms and sold to unsuspecting members of the public.
The RSPCA has launched a campaign to call on the UK Government to introduce laws in England to combat puppy dealers selling sick and dying animals.
It is calling for a clampdown including mandatory licensing for anyone selling puppies in England along with strong fines and penalties for anyone caught selling a puppy without a licence.
There should also be a licence fee-funded national database of puppy sellers to aid enforcement.
The RSPCA also wants all internet and offline advertisers to display the licence number of the seller to list an advert.
Mr Briggs said: "We are hearing from more and more people across the country who have bought puppies that have fallen ill or died.
"It is heartbreaking. These puppy dealers have no care or consideration for the puppies they are trafficking in and then selling or for the families they are selling these poor animals to.
"Many are kept in awful conditions in 'holding pens' before they are taken to set-ups made to look like family homes to be sold on to the public.
"The people buying them often have no idea the pup they have taken on is sick or, in many cases, dying - until it is too late."
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