Almost a third of puppies bought online die or fall ill within their first year, a charity has warned.
Research found 18 per cent of people who bought their pet directly over the internet after finding an advert online said their dog had been sick before its first birthday, with the illness continuing throughout its life or until its death.
A further 13 per cent said their puppy had fallen ill in the first 12 months but went on to recover.
The survey, carried out by the Kennel Club, comes after the Government announced plans last month to ban the sale of puppies and kittens by pet shops and other commercial dealers.
The charity warned that puppies were being put at risk by a culture of "instant gratification".
The poll of 2,176 dog owners also found that one in three people do less than two hours' research when buying a puppy, while 12 per cent of people - equating to roughly a million dogs - pay for their puppy before they have even seen it.
An estimated 630,000 puppies among a dog population of nine million were ordered online and delivered directly to people's homes.
Caroline Kisko, secretary of the Kennel Club, said: "The internet is making it easier than ever before to buy things instantly, and this is having an alarming impact on the way people expect to buy a puppy.
"A shocking number of people are spending less than two hours researching their puppy purchase and this is leading to a serious welfare crisis.
"Rogue dog breeders selling directly to puppy buyers can be masking terrible conditions and the yawning gap in puppy buyer awareness about how to identify a good breeder leaves people - and dogs - very vulnerable."
The charity said it was particularly concerned about so-called "fashionable" breeds, such as cockerpoos, warning that "bad breeders" would be "only too happy" to fill a gap in the market.
Last month, the Government announced plans to help reduce serious health problems and socialisation issues which afflict pets kept in poor conditions by unscrupulous, profit-driven breeders.
Anyone buying or adopting a pet less than six months old will have to deal directly with the breeder or a rescue centre under the proposed ban.
Ms Kisko added: "The government's plans to ban the third party sale of puppies, through pet shops and the like, is hugely welcome but puppy buyers shouldn't become complacent."