The Government must take urgent action to stop under-age puppies and heavily pregnant dogs being smuggled across Europe for sale in the UK, campaigners said.
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An undercover investigation by the Dogs Trust found animals were being forced to travel more than 1,000 miles across Europe in "shocking" conditions to meet the demand for fashionable breeds of dogs.
The Dogs Trust said that as the UK leaves the EU, the Government should redraw the non-commercial pet travel rules, which are being exploited to smuggle puppies to the UK from central and eastern Europe.
Since the pet travel scheme, currently governed by the EU, was changed to allow puppies to enter UK at a much younger age, unscrupulous dealers are using the opportunity to make millions of pound from illegally importing them.
Puppies are being sedated in cramped conditions for more than 30-hour journeys, while pregnant dogs are being forced to travel to allow the puppies to be born in the UK, the charity's investigation found.
Dealers claim to evade border controls across Europe, while hundreds of dogs are producing "designer" breeds of puppies including French bulldogs, some of which are advertised for sale in the UK.
The charity also claims corrupt vets are falsifying pet passports, providing fake vaccination stamps to allow puppies to travel when younger than allowed, and supplying sedatives to help smuggle them into the UK without detection.
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The Government should reduce the number of dogs allowed under non-commercial movement rules, and require visual checks of all dogs entering the UK, the Dogs Trust said.
The charity said there should also be a centrally accessible database logging pets' microchip numbers and date of entry to the UK, maximum penalties for those caught illegally importing dogs should be increased, and there should be specific requirements for the welfare of dogs during transport.
The Dogs Trust is also asking members of the public to contact their MP via the charity's website to help put an end to the puppy smuggling trade.
Paula Boyden, veterinary director of Dogs Trust, said: "Our investigations have revealed shocking welfare conditions.
"There is no regard for the lives of these vulnerable puppies, who are subjected to gruelling journeys of more than 30 hours in hot, cramped and filthy conditions with no toilet breaks and very little water.
"Some don't survive the journey, and those who do may have physical health and behavioural issues due to their poor start in life."
She said puppy smugglers were only concerned with making a profit, importers were exploiting the lack of visual checks at borders and insufficient penalties meant there was no real deterrent for illegally importing animals.
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"With Brexit around the corner this is an opportunity of a lifetime to put robust measures in place that protect dogs and the public," she urged.
A Department for Environment, Food and Rural Affairs spokesman said: "This abhorrent and illegal trade is cruel and unnecessary, and risks spreading very serious diseases such as rabies.
"We have one of the most rigorous pet border checking regimes in the world and we are planning to crack down further with a complete ban on third-party puppy and kitten sales.
"This will mean the people who have a complete disregard for pet welfare will no longer be able to profit from this miserable trade.
"Leaving the EU will also give us the opportunity to bring in further controls to stop this sick trade."