More than £5m in unpaid tax has been recovered from hundreds of dog breeders and traders selling puppies on the black market.
HM Revenue and Customs (HMRC) launched an investigation in 2015 after animal welfare groups expressed concerns puppies were being reared on a mass scale and sold illicitly in the UK.
It has recovered a total of £5,393,035 in lost taxes from 257 separate cases.
Several arrests have also been made during the four-year investigation.
Among those HMRC said were targeted by its special taskforce were two unconnected puppy breeders in the West of Scotland who were handed tax bills of £425,000 and £337,000.
A former Crufts judge breeding puppies in the Midlands was found to owe £185,000 in unpaid taxes, according to the government agency.
A dealer in Northern Ireland was told to pay £185,000 in tax while a Somerset breeder was given a £114,000 bill, and a puppy breeder in Swansea was handed a £110,000 tax bill, it said.
Financial Secretary to the Treasury, Mel Stride, said: “It is utterly appalling that anyone would want to treat puppies in such an inhumane way and on such a scale.
“It’s also deeply unfair to all of the legitimate businesses who do pay the right tax, and the total recovered by the taskforce is equivalent to the annual salaries for more than 200 newly qualified teachers.
“We continue to work hard with other government agencies and our partners to tackle these traders.”
HMRC is also involved in a multi-agency collaboration across the UK and Ireland called Operation Delphin, which is designed to tackle illegal puppy smuggling and its consequences.
It is led by the Scottish SPCA and includes partners such as the RSPCA, Ulster SPCA, Dublin SPCA, Irish SPCA, HMRC, Border Force, and the police.
As part of Operation Delphin’s activities, the SSPCA said it had seized 27 puppies smuggled from Ireland at Cairnryan Port in Dumfries and Galloway.
The RSPCA said it had intercepted 96 puppies being smuggled into the UK overnight through Holyhead Port in Wales, in a shipment it said was likely to have been worth “many tens of thousands in murky profits”.
Claire Lawson, from the RSCPA, said: “Sadly, people are readily prepared to act illegally and compromise the welfare of defenceless animals to make profit.
“That’s why the work of the HMRC taskforce in tackling dishonest dog breeding practices is so important.
“Puppy breeding can be big business, and those seeking to sacrifice their welfare to make more money need to know that this behaviour will not be tolerated, and they will punished.”