Weapon dogs being traded online for Playstations by criminal gangs.

Fighting dogs bred in London are being traded online by criminal gangs in return for PlayStations, animal campaigners have warned.

The Pet Advertising Advisory Group said it had found illegal breeds including pitbull terriers on sale in online classifieds site Gumtree and on social networking sites.

The dogs are often trained to kill by their owners and used as a status symbol among fellow gangsters - and as a warning to rivals.

One London-based seller posted an ad offering a "fighting dog with big teeth". Another owner had a Staffordshire bull terrier on the market at £120.

The advert read: "I don't mind swapping for a PS3, XBox 360, Nintendo Wii, laptop, moped."

Clarissa Baldwin, chairman of PAAG, told the Guardian: "Our research has revealed some truly terrible examples where animal welfare was clearly the last thought in the mind of the advertiser. Every day we hear from people who have bought an animal online only for it to fall sick or die soon after."

Battersea Dogs and Cats Home said the number of abandoned dogs had increased as a result of the trading. A spokesperson said that some owners dumped their dogs if they failed to sell or swap them online.

The group urged Londoners to report cases of fighting or illegal dogs being traded to police.

According to a study by criminologist Dr Simon Harding, underworld puppy farms housed in flats and garages breed "status" and weapon dogs worth thousands of pounds.

The Middlesex University lecturer said: "It's this idea some young men have that the dog has got to bring a credible threat of violence. To do that, they have to be sufficiently aggressive. If they are not, it will bring ridicule on the owner among their peer group."