Battersea Dogs & Cats Home wants those seeking to own a pet to come to them rather than buying animals off the internet.
An advert for a new dog is put online every two minutes, while those for cats is placed every four minutes, a sign of how many potential pets are for sale.
At Battersea, however, they are struggling to find owners to adopt the animals they house due to the number of pets placed for sale online and social media, many lacking rigorous regulation.
In 2017 Battersea rehomed 8 per cent fewer dogs and 16 per cent fewer cats than 2016 as more people turn to the internet to find a new pet.
Battersea's Deputy Chief Executive, Peter Laurie, said: "Our dog rehoming has been declining for years - last year we rehomed 8% fewer dogs than 2016. Now we are seeing similar trends for cats too. We were rehoming growing numbers of cats until last year when numbers fell 16%, as we and most animal charities struggle to compete with the mammoth platform of online sales.
"You can buy a pet with the click of a mouse, often with no idea if you're going to get the same animal as the cute photograph that caught your eye and with no real idea of its background or personality.
"It's animal rescues that then pick up the pieces. In the first three months of this year alone, we've taken in more than 100 dogs bought online, whose owners can no longer care for them, often due to veterinary problems or behavioural issues they didn't know they had."