HMRC has raised over £5m in unpaid tax after an operation to tackle dog breeders who were selling puppies on the black market.

More than 250 cases involving the illicit sale of dogs were uncovered by the taxman during the four years after a special taskforce was formed to tackle the problem. In one instance, a former Crufts judge was given a £185,000 bill.

Among those who were fined were two unconnected breeders in the west of Scotland who were given tax bills of £425,000 and £337,000. Another dealer in the east of Scotland was hit with a £400,000 demand.

The inquiry started after animal welfare groups told HMRC that thousands of animals were being bred in unregulated conditions and sold on the black market.

“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,” said Mel Stride, financial secretary to the Treasury.

Several arrests have been made as a result of the operation. Kevin Knox, a kennel owner from Durham, pleaded guilty to tax evasion last year.

The tax bills were issued to dealers and breeders across the UK. An operation to crack down on illegal cross-border puppy smuggling is also taking place and includes Irish agencies. Thousands of dogs come from Northern Ireland and the Republic of Ireland after being bred on large scale puppy farms. The Scottish SPCA seized 27 puppies at Cairnryan port in Dumfries and Galloway which were being smuggled from Ireland.

Anyone with any information surrounding the illegal import of puppies has been asked to report it to HMRC.