Unsocialised dogs may become more dangerous and other large breeds could replace XLs, says vet.
The ban on XL bullies could lead to more dog attacks, a senior vet has told the BBC.
Dave Martin, a welfare advisor to 900 practices in the UK, said attacks in the home could increase if the ban leads to more XLs being kept indoors without stimulation and exercise.
Two dog charities also said they feared irresponsible breeders would move to other large dogs to get around the ban.
The UK government said its ban would protect the public.
The ban on the dogs in England and Wales will come into force on 31 December.
Owners who wish to keep their dogs must apply to the exemption scheme or they can choose to have their dog euthanised and apply for compensation.
The daughter of Shirley Patrick, 83, who died 17 days after being mauled by an XL bully cross, said the ban should go further and include other breeds.
Mr Martin, a vet of 26 years, said he understood why the UK government had taken action, but was concerned about unintended consequences of the ban, which requires XLs to be muzzled and kept on a lead in public.
“My worry is that the American bully XL population is quite young… so we’ve got young, fit, athletic dogs who are going to have a high demand for exercise, high demand for stimulation,” he said.
“We know a lot of the fatal attacks have occurred in the house or with dogs escaping… if we end up frustrating these dogs, they’re not getting the exercise they need, is that going to run the risk of more attacks within the house?
“We don’t know is the simple answer but it is a concern…some dogs may be more likely to attack than they were beforehand.”
“My worry is that the people… who want an aggressive dog for whatever reason go, ‘well, I’m not going to have a bully XL anymore, I’m going to have something else’.”
He said large, muscular dogs such as the cane corso and the presa canario could become the next status dogs.
BBC Wales Investigates spoke to one breeder, who did not want to appear on camera, who said he had already moved on from selling XL bully dogs, to another large breed – the Caucasian shepherd.
He described them as being “larger, more powerful and potentially more dangerous in the wrong hands” than XLs.
Grandmother Shirley Patrick was fatally attacked by an XL bully cross at her home in near Caerphilly in December 2022.
Her daughter, Gail, said the dog was brought into the house by someone else.
“The dog just ran from the conservatory… into the living room and went straight for her face,” she said.
Gail has since joined a campaign for responsible dog ownership along with the mother of 10-year-old Jack Lis, who was also killed by an XL bully.
She said she was also concerned about other breeds, and has written to the prime minister to outline why she feels the current ban did not go far enough, and is calling for all large dogs to be muzzled in public.
“Somebody has got to do something about this,” she said.
“It’s only going to be a matter of time [before] another monster breed will be bred.
“I don’t want my mother to have died in vain.”
Rachael Millard, a mental health nurse, was attacked by two XL bullies in March 2022 while trying to protect her 15-week-old Boxer puppy, Floyd.
The attack has left her on medication for post-traumatic stress disorder (PTSD) and unable to work as much as she did.
“I had three puncture wounds on the left side of my leg and I had a puncture wound on the right side and I had a chewed-up finger,” she said.
The owner of the dogs was jailed for 18 months after admitting they were dangerously out of control – and was banned from keeping dogs at the time.
Despite her experience, Rachael is not in favour of a ban, and instead wants the government to look more closely at regulating owners.
“You can’t just penalise a breed, I think that they should look at aggression in all breeds of dog… to have some type of assessment then to see who can have and handle those dogs,” she said.
Jayne Shenstone runs German Shepherd Rescue, which also rescues Caucasian and Central Asian shepherds.
She said: “I think these could potentially be far worse than an XL bully… we’ve got ticking time bomb here, a real ticking time bomb… and I don’t want to see that happen to these magnificent dogs.
“Probably 95% of homes in the UK could not cope with the Caucasian.
“They probably have more weight, more strength… in the wrong hands and in the wrong situation, I think potentially a Caucasian could kill,” she said.
She said her rescue centre only walked their dogs wearing muzzles and only rehomed them to rural locations.
Image source, Facebook
How many XL bullies are there?
The UK government has suggested about 10,000 – but the BBC has discovered it could be at least three times that number.
One of the largest vet groups in the UK, IVC Evidensia, told the BBC Wales Investigates programme about 5,440 dogs described by owners as being XL bullies were registered with them.
The company has about 900 practices, which is about a sixth of the UK market.
If similar numbers of XL bullies existed in other UK vet practices there could be at least 30,000 of the dogs.
Many more thousands may be listed as cross-breeds or under a broader breed-type, the company said.
What do police figures tell us?
BBC Wales Investigates asked all 43 forces in England and Wales for information about the number of dogs seized and 24 responded.
Between 2019 to 2022 dog seizures increased by 51%, from 2,743 to 4,133.
It is not clear how many were XL bullies because police officers do not have to record the breed of the dogs they seize.
Of the 15 forces that provided data on the dog breeds – although they record them in different ways – Staffordshire bull terriers and German shepherds appear to be among the most seized.
If not a ban, then what?
Ms Shenstone said she did not believe dog licences would work because only responsible owners would sign up.
“Anybody can buy one of these dogs – you see them advertised on the internet… most of these breeders don’t have checks, they just take the money, and that’s it,” she said.
Image source, Family photo
Sara Rosser, a behaviourist with the Hope Rescue charity in south Wales, also feared irresponsible breeders could find ways around the ban.
She suggested they would only support licensing if it came with enough funding to enforce it properly.
“I think a lot of it is about better enforcement of the laws that we’ve already got in place – microchipping for example, here we only see a 25% compliance rate with the microchipping laws and yet there is no punishment or enforcement of that,” she said.
“That alone would help us be able to trace irresponsible breeders.”
The UK Government’s Department for Environment, Food and Rural Affairs said in a statement it had taken “quick and decisive action” to protect the public and was “making sure” existing powers to tackle dog control issues were applied.
It added it had also set up a Responsible Dog Ownership taskforce with the police, local authority representatives and animal welfare stakeholders.
Dogs on Death Row: Will the Bully Ban Work? will be on Monday at 20:00 GMT on BBC One Wales and iPlayer.