Bully Watch told Yahoo News UK that people are getting rid of their dogs due to a drop in demand for the controversial breed.
The home secretary has commissioned ‘urgent advice’ on outlawing XL bullies following attacks.
A dog shelter said the breed was “running amok” in the streets after owners abandoned them.
Breeders of American XL bully dogs are “simply abandoning them” due to recent negative press, a campaign group has claimed.
Calls for a ban of the breed have intensified since footage emerged showing a suspected XL bully on the rampage in Birmingham at the weekend, mauling several people including an 11-year-old girl.
Home secretary Suella Braverman has since announced she has commissioned “urgent advice” on outlawing the dogs.
Bully Watch, a group campaigning for the complete ban on the XL bully breed, told Yahoo News UK that they have seen evidence of “backyard breeders” getting rid of their dogs due to a collapse in demand.
A spokesperson for the group said: “What we see, is because the market price of XL bullies has collapsed due to a fall in demand and seemingly endless increase in supply, many backyard breeders are simply abandoning their breeding dogs - both studs and bitches.
“This has been going on for months now.”
Pages advertising the sale of XL bullies, seen by Yahoo News UK, show entire kennels being sold by breeders, which includes one female known as ‘The Tank’, for £15,000.
The XL bully debate: Read more
While breeders are shutting down their kennels and selling up, other XL bully owners are said to be abandoning their dogs in the streets, leading to some dog shelters being inundated with the breed.
One shelter, City Dogs Home in Bucknall, Stoke-on-Trent, said that it had seen evidence of “relentless” overbreeding, and that some dogs are winding up there after simply being cut loose by their owners.
City Dogs Home manager Vickie Phillips told StokeonTrentLive that “every other dog people bring in is an XL bully or American bulldog”, and said she believed she’d seen evidence of “relentless over-breeding”.
She said: “There are people who just throw them out on the streets and the dog wardens have to bring them in, putting people at risk with them roaming around aimlessly.
“They're big dogs and once they get someone on the floor, that's it.”
She backed calls for the breed to be banned, adding: “There are too many of them – they've been over-bred and now they're running amok.
Watch: XL bully - mother of four-year-old attacked by dog says breed should be banned
“It's every month you hear about (someone being attacked) and it seems more pronounced than it was with the Pit Bulls.
“We've had some nasty ones here. They can be aggressive to other dogs – and have been to people here at the kennels. They've tried to properly go for us, but we're obviously very cautious, so, thankfully, no-one's been injured yet.”
‘Most dangerous dog breed ever’
Phillips said she expects more XL bullies will end up at the shelter because of the home secretary’s call for a ban.
Veteran dog behaviourist Stan Rawlinson, 76, described the breed as “the most dangerous dog breed ever”, saying the animals were “predatory, over-reactive, stimulated by movement, distrustful of strangers and incredibly strong”.
But the leading website BullyScene describes XL bullies as being “gentle and unwaveringly loyal” and “the perfect match for families”.
What dog breeds are banned in the UK?
If calls for the XL bully to be made illegal in the UK are successful, it would make it the fifth breed to be banned.
Pit Bull Terriers, the Japanese Tosa, the Dogo Argentino and Fila Brasileiro are already outlawed in the UK – but it is not just owning them that can lead to a prison sentence.
Under the guidelines of the Dangerous Dogs Act, which came into effect in 1991, anyone who owns – or sells, abandons, gives away or breeds from – an illegal breed faces punishment.
Anyone with a banned dog can have it removed by the police or local council, even if there have been no complaints and the dog is not acting dangerously.
Police can also seize a banned dog without a warrant if seen in a public place – experts will then determine if the animal is an illegal breed.
Courts can determine that a banned dog is not a danger to the public and will allow an owner to keep it if put on the Index of Exempted Dogs (IED).
However, the dog must be neutered, microchipped, kept on a lead and muzzled at all times when in public and kept in a secure place so it cannot escape.