The government said it will roll out the legislation next year after holding public consultations that showed 95% support for the ban.
“This will mean that anyone looking to buy or adopt a puppy or kitten under six months must either deal directly with the breeder or with an animal re-homing centre,” the Department of Environment Food and Rural Affairs (Defra) said as part of its Christmas animal welfare push.
The measure is commonly called Lucy’s Law in honour of a Cavalier King Charles Spaniel that was rescued from a puppy farm in Wales in 2013.
Lucy, who died in 2016, spent most of her life in a cage and was unable to breed because of her lack of movement.
Activist Lisa Garner took her home and launched a social media awareness campaign that changed the way Britons get their pets.
The government said the new law would help "end the terrible welfare conditions found in puppy farming and solve a range of existing animal welfare issues".
‘Right start in life’
The government believes the ban will keep “high volume, low welfare breeders” – both licensed and unlicensed – from flooding pet shops with puppies and kittens raised in unethical conditions.
Defra released no figures estimating how many sales the new legislation would affect.
But London’s Battersea Dogs Home chief Claire Horton said the rules will “make sure the nation’s much-loved pets get the right start in life”.
Battersea has been the focus of several popular TV series about pet rescues and care that reflected Britons’ general affection for cats and dogs.
Britain’s People’s Dispensary for Sick Animals (PDSA) veterinary charity said 49% of UK adults owned at least one pet in 2018.
PDSA’s estimated cat population of 11.1 million edged out the 8.9mn dogs and 1mn rabbits – whose numbers have nearly halved since 2011.
The British government has unfurled a number of animal welfare initiatives in the past few months that activists hope other European countries will soon follow.
One law in October banned licensed shops from dealing in puppies and kittens under the age of eight weeks.
Defra is now looking at legislation requiring all non-commercial rescue and re-homing centres to have a licence.
It separately urged families to steer clear of the temptation to buying their kids cute pets as Christmas gifts.
“This carries a number of risks and can have tragic consequences for animal welfare when pets are either abandoned or given to re-homing centres to deal with,” Defra said. “Pets should not be bought as a surprise.”