One site blocked 40,000 adverts in just five months.
Pets4Homes said Staffordshire bull terriers, pomeranians, labradors and golden retrievers were the breeds most commonly the subject of blocked adverts. Puppies were up for sale unethically or offered in adverts likely to be fraudulent.
Puppies.co.uk described an “exponential” increase of suspicious adverts which saw them remove up to 200 adverts per week. A spokesman said: “[Blocking suspicious adverts] was a significant makeup of the working week.”
In April, the Government introduced Lucy’s Law – named after a puppy who was rescued by Warwickshire owner Lisa Garner after she was sold in a problem puppy farm.
It tightened controls on puppy breeders, meaning buyers can only purchase a puppy from the place they were born or a reputable rehoming centre.
But many supposed sellers ignore this.
And others offer puppies using fake pictures, taking the money but not handing over a dog. Many scams are carried out by organised criminals abroad.
The RSPCA is concerned even the new rules still allow breeders abroad to have a certificate to sell puppies as long as they are licensed to breed in their home country.
“There is no way of checking the conditions these puppies are kept in, some of which are detrimental to long-term health,” the charity said.
Pets4Homes now cross-checks IP addresses to prove vendors behind adverts live in the UK and at the address listed on their account.
But chief Axel Lagercrantz says the issue of fraudulent puppy sales is so persistent, his company introduced a safety deposit system to stop buyers paying until a puppy is delivered.
He said: “Our concern is that a significant proportion of the suspicious adverts we stop are still being utilised by would-be scammers in less regulated corners of the internet.”