Two puppies rescued from a cardboard box in a layby on one of the hottest days of the year have died.
The spaniels were found by a cyclist on Shopland Road in Rochford, Essex, on Saturday along with a third puppy that was already dead.
The RSPCA said the puppies could have been imported from abroad to sell and abandoned when they became unwell.
Samantha Garvey, the RSPCA's chief inspector for Essex, said the animals died from suspected parvovirus.
The canine virus is highly contagious and causes severe vomiting, diarrhoea and dehydration.
The cyclist who found the puppies, thought to be about eight weeks old, said he spotted them when one "peeked" out of the box.
Ms Garvey called the case "horrific", adding it was "possible" the animals were from a puppy farm.
"We see it a lot. Dogs are brought across because they are cheaper to source and import for sale from Europe than they are in the UK," she said.
"It's a problem of supply and demand. A lot of people are off work [because of the coronavirus pandemic] and think it's the perfect time to get a puppy.
"I would just ask that people really do their research and not agree to buy any animals who they have reason to believe are sick."
Anyone with information about how the spaniels came to be dumped by the side of the road is asked to contact the RSPCA.