The owners of American XL bully dogs may be able to keep their pets, despite the prime minister pledging to ban them following a series of attacks, the chief vet has said.
Rishi Sunak has said he will outlaw the animals under the Dangerous Dogs Act and new laws will be "in place by the end of the year".
There are concerns, however, that defining a breed could be difficult because the dogs are cross-bred.
The PM's announcement came after Ian Price, 52, was mauled to death by two dogs in Staffordshire on Thursday, in what police say was a suspected XL bully attack.
Professor Christine Middlemiss, the UK's chief veterinary officer, said arriving at a "consensus" on a definition will be one of the first things officials will do.
An "amnesty" approach would mean there would not be a cull of the dogs, Prof Middlemiss said.
"People that already have these dogs - and some of them will be well socialised, well managed, well trained - you will need to register and take certain actions," she told the BBC.
"Your dog will need to be neutered. It will need to be muzzled when out in public and on a lead and insured.
"But if you comply with these actions, and that means we'll know where these dogs are, which will be a massive benefit, then yes, absolutely, you will be able to keep your dog."
It is an approach which echoes that taken when pitbull terriers were banned under the Dangerous Dogs Act in the 1990s.
Lord Baker, who worked on the act, said XL bullys should be "neutered or destroyed" once the ban has come into force, with any permitted to live being "muzzled for the entire time".
He also said it should be done "almost immediately because this is a very dangerous breed and it has actually killed children and attacked other people".
The Tory peer told LBC: "This dog is, in fact, bred in order to fight and to be aggressive. It has already done enough damage."
Groups including the RSPCA and the Kennel Club have said banning XL bullys will not stop attacks.
But Lord Baker said he did "not accept" those views.
Legal academic Dr Lawrence Newport said bans make a difference, and one has been "in place on pitbulls since 1991".
He told Times Radio: "That ban has been very successful. And we know that because for example, in the UK, we have half the per capita deaths to dogs that the US does, and that difference is entirely explained by pitbulls."
Labour leader Sir Keir Starmer said he supported the ban but accused the prime minister of "dithering".
He told broadcasters: "There has been a clear case for banning them for a long time. What I say to the government is good, get on with it, and the sooner we can do this the better."
Police in North Wales said five people were reportedly bitten by a dog at Palins Holiday Park, Kinmel Bay, on Friday evening. "A man with a serious arm injury was subsequently taken to hospital," officers said.
That dog is not believed to be an XL bully.