Puppy Maggie had been with the family for less than 24 hours when she began to show signs of sickness. Grace Piper from Worthing, West Sussex, had picked up the eight-week-old from a breeder in Brighton that advertised its puppies on pets4homes.co.uk for £750 each in cash.

“She started being sick, she had diarrhoea, was lethargic, dehydrated and withdrawn,” Piper told the Guardian. “We called the vets [and] were told to feed her chicken and rice and hope for an improvement. All the time we were encouraging her to drink or eat something but all she wanted to do was sleep. We were increasingly worried.”

Then Maggie collapsed on the floor. “At this point we knew something was seriously wrong with her. We called the vets again, and were told to bring her in and all of a sudden she stopped breathing. We gave CPR and mouth to mouth … The vets rushed her in to perform CPR, but after five minutes we were told that they had done everything they could, but that she wasn’t waking up.”

Piper is one of a number of people who bought puppies online from the same location in Brighton. The Guardian has spoken to two people, both of whom paid large sums of money for the puppies before losing them to canine parvovirus, a highly contagious disease which causes acute gastrointestinal illness.

The deaths come as the RSPCA says that complaints about illegal puppy farming have increased dramatically over the past 10 years and that 87% of current puppy trade calls are about animals purchased online.

“The owners who sold us Maggie would have most likely seen symptoms before we got her but still sold [to] us anyway. The mother should have been vaccinated but clearly was not,” Piper said.

“They were in a rush to get rid of her, and as soon as we left the door their phone was off, and we could no longer contact them.”

Piper, who shared her dilemma on Facebook, heard from other owners who said they had also bought puppies from the same dealers. When she spoke to the RSPCA, they told her that their “inspector is going round to take those dogs out of the house as they are infected and should not be sold”.

An RSPCA spokesperson told the Guardian that they were aware of the Brighton breeder in question but said: “Unfortunately, we can’t discuss complaints related to specific people or addresses and what action may have been taken.”

“Sadly, we hear stories like this all too often. This is why we urge people to be extremely careful when buying puppies from breeders.”

The RSPCA received 4,357 complaints in 2018 about the puppy trade in England, compared with 980 reports 10 years earlier in 2008. It marks an almost five-fold increase. East Sussex has seen a 127% increase in such reports in a decade.

“Looking back after what’s happened to us,” said Piper, “it’s made us realise the house was strange. No toys and the kids weren’t allowed near the puppies. But we assumed then that the kids were kept away at the time for the viewing.”

Isabel Winters, who lives in Brighton, bought the last of four labrador puppies advertised by the same breeder on Gumtree. It fell ill within a few days before dying in a veterinary hospital due parvovirus.

“The whole thing is clearly a scam,” she said. “I bought it from Gumtree and thought it was with a pedigree breeder. I thought it was ok.”

Winters’ purchase was a present for her son. “My son’s 14th birthday was coming up. He lost his father when he was nine, so this was a big thing for my son, to have a puppy. It was going to be a Christmas present and birthday present all rolled into one.”

Puppy and kitten farming to be banned under 'Lucy's law' Read more

Winters said she wanted to visit the breeder’s location to check on the puppy’s mum and dad, but “they were very vague, they gave me a street name but not the number of the house”. Instead, they brought the puppy and the mother to her door. “It was all very sudden and quick. They arrived and they looked like a pair of charlatans.”

Winters paid them £400 cash and arranged to pay the rest via online banking but her internet was down, so the dealers came in person for the money the next day.

New government legislation is expected to come into force in England in April, banning the sale of puppy and kittens from third parties, meaning that owners will need to deal directly with breeders or rehoming centres. It is known as Lucy’s law after a Cavalier King Charles spaniel who died in a Welsh puppy farm due to constant forced pregnancies and dirty and cramped conditions.

Illegal puppy farms work under the radar and avoid regulation or inspection, says Bill Lambert, senior health and welfare manager at the Kennel Club. “They are driven by profit and, as such, ignore animal welfare legislation or even basic welfare guidelines. Their breeding bitches will often be bred litter after litter, they will receive little human contact and will rarely be socialised,” he said.

“Puppies are often kept in poor conditions where even basic, routine health measures such as health screening, immunisation or worming are applied. These puppies will often be taken away from their mothers far too early, transported away from where they are born and raised and sold to on to unsuspecting owners.”

Increasing use of the internet to buy puppies has exacerbated the issue. Research by the Kennel Club last year found that about 30% of puppy buyers purchased their dogs from a puppy farm, a rise of 7% compared to 2018. “The growth of the internet has changed the buying habits of a lot of people and illegal puppy farmers will use advertising websites and social media to hide their true identity and use a website that makes them appear respectable and trustworthy when the reality may be completely different,” said Lambert

Lambert said parvovirus attacks the intestines and immune systems of affected puppies and dogs, and can be deadly if left untreated. “It is highly infectious and is passed from puppy to puppy very quickly. Initial signs include a high temperature, having less energy and being off their food, with later symptoms including vomiting and diarrhoea.”

Daniella Dos Santos, the president of the British Veterinary Association, said: “Vets see first-hand the tragic consequences resulting from poorly-bred puppies, who are often taken away from their mothers at a very young age. They often suffer from disease, health problems and poor socialisation.”

In November, the parliamentary environment, food and rural affairs committee wrote to the government expressing concerns about its commitments to stopping the illegal puppy smuggling trade. “The evidence we have heard so far has highlighted both the scale and the profitability of this cruel illegal trade,” said the committee’s chair, Neil Parish MP.

Before buying puppies, Lambert said, buyers should make sure to see the puppy in its home environment, interacting with its mum, and also see paperwork such as vaccination records. He strongly advised against buying a puppy from a neutral location such as a motorway service station or a car park and said buyers should be wary of breeders who insist on purchase on the buyer’s first visit.

Axel Lagercrantz, Pets4Homes’s CEO, said: “We were incredibly sad to hear the news of Maggie the labrador, and our thoughts are with Grace and her family. Our team of support staff works around the clock, seven days a week to monitor the website. Grace was of great help as she contacted our support staff when she realised something was wrong, and we were able to block the advert and the user from our platform promptly.”

“For anyone considering adopting or rehoming a pet, we recommend consulting the advice section on our website, and the thorough checklist that we encourage every pet buyer to follow. ”

A Gumtree spokesperson said: “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. We work hard to deter unscrupulous operators on the Gumtree platform through our compulsory paywall, and will delete any listings that we believe indicate signs of animal cruelty.

“We encourage users to watch out for people selling popular breeds for cheap prices, to always check for medical records and to never purchase a pet before going to see it in person. If the conditions the animals are being kept in are suspicious, report the seller to the RSPCA and Gumtree immediately.”