South Yorkshire has been named as a puppy farming hotspot based on the number of calls the RSPCA receives about suspect breeders and dealers.
The RSPCA had its busiest year to date tackling the underground puppy trade in 2016, receiving more calls than ever before from people reporting concerns.
It received 3,688 calls - up six per cent on the year before and 132 per cent more than five years earlier.
Almost 90 per cent of calls were from people who bought puppies online.
Off the calls,136 - 3.5 per cent of the overall total - were made about South Yorkshire sellers.
Last year there were 204,182 listings for dogs posted on the online selling site Gumtree, with 5,189 - 2.5 per cent - posted by South Yorkshire sellers.
RSPCA dog welfare expert Lisa Richards said: “We urge anyone thinking of buying a puppy to be extremely careful when they’re choosing what type of dog to buy, where to buy from and who to buy from.
“It’s so important to do lots of research not only into the type of dog you want and the kind of care he needs, but also into any potential breeder that you’re considering going to.
“Today, particularly with the rise of puppies being advertised and sold online, it’s becoming more and more difficult to tell the responsible breeders apart from the unscrupulous ones.
“We would encourage anyone who is thinking of getting a puppy to use the Puppy Contract to help ensure they buy a happy, healthy dog.”
Lisa added: "We want all sellers to put the welfare of their dogs and puppies ahead of everything else but, unfortunately, there are many dealers who care only about their profits at the expense of the animals’ health and welfare.
“While classified websites, if used responsibly, can be an effective method of advertising for responsible breeders and rescue organisations, sadly, far too many people abuse the internet and this has led to many animal welfare issues arising.
“As well as the classified websites, like Gumtree, working with us to crackdown on these dealers, we also need the public’s help to stamp out this underground trade. So please take care when buying a puppy. We know it’s hard but if you’re concerned about anything when you visit a breeder, walk away and call the RSPCA and your local authority.
Morten Heuing, general manager at Gumtree UK, said. “Whilst most people have safe and successful experiences rehoming dogs on Gumtree, there are a minority of unscrupulous breeders and dealers who put profit ahead of the health and welfare of the animals,”
“We take the welfare of animals very seriously and work hard to ensure our site is a safe place to find pets in need of rehoming. Our dedicated safety team takes steps to make it as hard as possible for illegal traders to operate on the site.
"For example, we do not allow ‘wanted’ ads in our ‘Pets’ category and we delete any ads we believe are encouraging or indicate signs of animal cruelty. If users have concerns about an ad on our site, they can use the ‘Report an Ad’ button on our website and our safety team will then investigate it."