XL bully dogs must be kept on a lead and muzzled when out in public – as new rules take effect from today.
Selling, breeding, abandoning or giving them away is also now illegal.
People have until 31 January to apply for an exemption certificate to keep their dog – and must have it neutered, microchipped and insured.
Owners in England and Wales who don’t get an exemption by then will have to euthanise their dog or face a possible criminal record and fine.
Thousands of people already been granted an exemption, the government told Sky News earlier this month.
The new rules come after a series of attacks in which people have died or been injured by the dogs.
In September, the prime minister declared XL bullies a “danger to communities” after a man died in a suspected attack in Staffordshire.
However, rescue centres recently warned at least 246 dogs faced being put down if they couldn’t be rehomed by the end of 2023.
Despite being able to apply for an exemption, the dog would have to live its life in a kennel, which charities say they would never choose for welfare reasons.
Samantha Gaines, from the RSPCA, said some owners might not be ready for the new rules as they had been brought in quickly.
“There is some fear that people for whatever reason may have left it a bit late and about what that means,” she said.
Ms Gaines also said it was important to use existing laws to tackle “root causes” of aggressive dogs, such as those who exploit and irresponsibly breed them.
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XL bullies were added to the Dangerous Dogs Act on 31 October, giving owners two months to prepare for the restrictions.
Environment Secretary Steve Barclay said the government had taken “quick and decisive action to protect the public”.
He said his department would “continue to work closely with the police, canine and veterinary experts, and animal welfare groups” as the restrictions come in.