American bully XL dogs in England and Wales must be kept on a lead and muzzled in public now that a new law has come into effect.
Breeding, selling or abandoning them is also illegal as of New Year's Eve.
Owners who want to keep their bully XL have until midnight on 31 January to apply to do so before an outright ban comes into effect.
The curbs follow a number of attacks, but campaigners insist banning the breed is not the answer.
Environment Secretary Steve Barclay said the government had met a pledge to take "quick and decisive action" following a series of attacks, with one man dying after being attacked by an American bully XL this year.
The breed has been added to the list of dogs banned under the Dangerous Dogs Act 1991.
To get a Certificate of Exemption, allowing owners to keep their bully XLs, they need to get insurance, neuter their dog, and pay a fee of £92.40.
The guidance is similar to that issued for the four breeds which have already been banned - the American pit bull terrier, the Japanese tosa, the Dogo Argentinos and the Fila Brazileiro.
But the RSPCA warned of a "huge risk" that rescue centres and vets will be unable to cope with a likely surge in demand.
Speaking for the charity, Samantha Gaines told the PA news agency: "What is really concerning is because the ban has come in at such a pace that there may be owners who are not ready for this, being able to ensure their dog is happy wearing a muzzle.
"There is some fear that people for whatever reason may have left it a bit late."
She added: "Breed is not a good or reliable predictor of aggressive behaviour."
Dr Gaines suggested that existing laws should be used more effectively to target people who exploit and irresponsibly breed the animals.
Many other animal charities have opposed a ban.
The Dog Control Coalition - which includes Battersea, Blue Cross, the Dogs Trust, BVA, the Scottish SPCA, the Kennel Club and Hope Rescue - say breed-specific bans have been proven to be ineffective.
The United Kennel Club in the US has said that an American bully XL "makes an excellent family dog".
"Despite its powerful appearance, their demeanour is gentle and friendly," it said, but also noted that "dog aggression is characteristic of this breed".
The new guidance does not apply to Northern Ireland or Scotland.
If Scottish ministers agree, then the ban will be applied in Scotland.
In Northern Ireland a ban on an American bully XL would require a separate change to legislation, either by a sitting Assembly and Executive or through an intervention by the Northern Ireland Secretary.
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