The RSPCA has warned thousands of American XL bully owners are set to breach new laws banning the dogs.
From 1 February in England and Wales, police can seize the dogs and owners could face a criminal record and an unlimited fine.
Owners are being urged to register dogs for exemption certificates or euthanise them.
More than 30,000 XL bullies have been registered - but the true number in the UK is thought to be much higher.
The deadline to apply online for an exemption certificate expires at noon on Wednesday.
People choosing not to keep their XL bully must take them to a registered vet to be euthanised.
The UK's chief veterinary officer Christine Middlemiss has urged people not to risk "leaving it to the last minute".
The government says XL bullies have been "disproportionately involved" in the rising number of fatal dog attacks.
In September, Ian Price died after being attacked by two suspected American bully XL dogs in Stonnall, near Walsall, while Jack Lis, a 10-year-old boy was killed by an American XL bully in Caerphilly in 2021.
But Dr Sam Gaines, a dog welfare expert for the RSPCA, said the organisation did not want the XL bully to be added to the banned list.
She said it would rather see a clampdown on irresponsible breeding, and targeting of individual dogs and owners putting people at risk through their behaviour.
Veterinary professionals have suggested there are about 40,000 to 50,000 XL bullies, with others putting the figure as high as 100,000, she added.
Dr Gaines said: "There are definitely going to be owners of XL bullies who will have not gone through the exemption process and obviously that is a concern.
"This is why we need to make sure resources are focused on individuals who are deliberately keeping dogs because they want to threaten people with them, frighten people and intimidate them."
Vets will have no legal or professional responsibility to report XL Bully breed types or their owners to the authorities, the Department for Environment Food and Rural Affairs (Defra) says.
"We would recommend that vets signpost owners to information about the ban and the actions that they should be taking where they suspect that a dog may be an XL Bully type," a Defra spokesperson added.
'Heartbreaking ban'
Ellie Lonsdale, 23, from Featherstone, West Yorkshire, has called on the government to stop the ban, describing her XL bully Frankie as "the softest thing".
"It's heartbreaking. What if I couldn't afford it like some others can't? It's not just an animal, it's family," she told the OceanNewsUK's 5 Minutes On podcast.
"Any dog can hurt anyone - it's not one specific breed, [it's] if you bring it up wrong. I don't get why people are saying it's just XL bullies."
She said Frankie had been properly trained and is "really kind and loving".
"His best friend is a little dachshund dog," she said.
"I understand why people are scared of him - he's big and muscly and got a big scary muzzle... but if you got to know him you'd know he's not [scary]," she said.
When the ban was announced the government launched a compensation scheme for XL bully owners to have their dogs put down.
People are eligible to receive £200 if they paid for the euthanasia or £100 if they used a free charity service.
Figures released under Freedom of Information laws show 117 compensation claims had been received as of 8 January, with £13,600 paid out.
Owners registering to keep their XL bully must comply with restrictions to ensure they are not a danger to the public.
The dogs must be microchipped, kept on a lead, muzzled in public and neutered to ensure they cannot continue breeding.
The breed is not recognised by the UK Kennel Club but the government has published guidance to help identify an XL bully.
Since 31 December last year, it has been illegal to keep an XL bully off the lead or unmuzzled in public, as well as breeding, selling or abandoning them.
Scotland has vowed to introduce similar restrictions on XL bullies at a later date, after an influx of dogs being rehomed there ahead of the English ban.
A decision on whether to ban the dogs in Northern Ireland is expected shortly.
Ms Lonsdale said the responsibility should be on owners to ensure their dogs are well trained.
"If you aren't willing to get the training for the dog, then you shouldn't have the dog," she said.
