LONDON (Reuters) - Something sinister is troubling Britain's usually genteel world of dog-showing, and at the center of it all lies one question: Who called Mrs Joyce Mann a "puppy farmer?"

Hate mail is circulating, threats have been made over the phone, Mann -- the country's top show judge -- has resigned and The Kennel Club says it has never seen anything like it.

"It is quite a nasty business," said a club spokeswoman. "Someone has set out to blacken Joyce's name at the pinnacle of her career."

And all this with the premier date on the dog show calendar fast approaching: Crufts, the world's largest dog show and the quintessence of British canine culture starts on March 4.

Joyce Mann, this year's Crufts Best of Show judge and wife of its chairman Peter, had been targeted by a fax campaign to highlight her mass breeding of Yorkshire Terriers in the 1960s -- puppy farming -- a practice common at the time, but now considered unethical.

"This has all the ingredients of a Miss Marple mystery," said Beverley Cuddy of "Dogs Today" magazine. "Dog showing is still very much a gentle middle-class pursuit, but it does have this other side.

"When people get obsessed, only winning counts, and the dog becomes irrelevant," she added.

"There have been poisonings over the years, and last year there was even a whispering campaign against Crufts winner Danny the Pekinese -- did he, or did he not, have a face lift?" she said. Danny was eventually cleared.

At first it appeared Joyce Mann might weather the smear campaign.

She released a statement last month saying she wanted to put the record straight: "In the late 1960s and early 1970s, I bred a considerable number of litters."

But, noted the British show breeders periodical "Dog World": "the gossip refused to die down, and at Manchester and the Pedigree Stakes finals, the Manns were the main topic of conversation."

It was all too much. Last week, Mrs Mann tendered her resignation, citing "stress and anguish as a result of scurrilous anonymous accusations, which have resulted in my receiving medical attention."

Her husband Peter followed, resigning from his post as Crufts chairman, a gesture of solidarity the Kennel Club accepted with "deep regret."

But tea-cups are still rattling. Who, after all, was responsible for this outrage? Dogs Today's Cuddy deduces the culprit might be a competitor who feared facing Joyce Mann in the show ring this year.