Police have seized 22 dogs in Sheffield, with most believed to be XL bullys.
The breed was banned in England and Wales in February, and owners had to apply for an exemption certificate.
Last month, campaigners were given the green light to bring a High Court challenge against the government’s ban.
South Yorkshire Police say they found the dog breeding facility in Sheffield on Thursday.
Officers say the dogs, including puppies, were living in “appalling conditions” at allotments on Infield Lane.
Dog legislation officers and the RSPCA also attended to support the police.
Six of the dogs had to be put to sleep immediately “to end their suffering”, the force said.
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Chief Inspector Emma Cheney said it was against the law to breed XL bullys since the ban came into force.
“Separate to the investigation around these dogs being XL bullys, animal cruelty and suffering will not be tolerated within South Yorkshire,” she said.
“The remaining 16 dogs remain in police kennels while our investigation continues.”
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Police say a suspect has been identified and enquiries are continuing.
The public have been asked to get in touch with any information they might have.
