Four dangerous dog breeds, illegal since 1991 to own, breed, abandon and sell, have been sold online with the use of fake breed names or code words.
The Pit Bull terrier, Japanese Tosa, Dogo Argentino and the Fila Brasileiro were made illegal under the Dangerous Dogs Act 1991 but have been sold online via classified sites across the UK.
Experts have warned "long legged", "game" and "blocky" are some of the code names to signal the true identity of the dog, the Liverpool ECHO reports .
Those selling these dangerous dogs have given them fake dog breeds including Irish Staffordshire terrier and the American Staffordshire terrier - breed which are not recognised by the Kennel Club or the RSPCA.
Animal charity, League Against Cruel Sports, which campaigns against dog fighting, said trading banned dogs happens on social media too.
On behalf of the charity, Emma Sobhe said: "It's so underground and secret and because of that it does happen online in closed groups.
"You have to have that element of trust to get in."
The profits made from breeding and selling illegal dog breeds is believed to be used in organised crime and even human trafficking.
The sales of dangerous dogs online is not necessarily linked directly to dog fighting, but the charity are campaigning to increase the minimum sentence for owning a banned breed or using them for dog fighting from six months to five years.
Animal welfare organisation group Pet Advertising Advisory Group (PAAG) is monitoring the situation too.
A spokesman said: "PAAG is aware that unscrupulous breeders use online classified websites to advertise illegal breeds or species."
"This is not a problem limited only to dogs, but is seen across the board with pets advertised for sale online.
"PAAG continues to work alongside a number of engaged websites to try to protect buyers from this type of illegal activity, which is often disguised through the use of key words.
"We would like to see our minimum standards for online sales become a legal requirement, in order to tackle this unethical trade."
Kennel Club secretary, Caroline Kisko, told the ECHO these online sites have now made it "easy" for people to buy and sell in order to "get whatever dog you want".
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