WHEN John met Judith, it seemed they were a perfect match. Both were devout Christians. He was a widower, she a divorcee. After finding each other on an online dating site, they exchanged heartfelt e-mails. John, an interior designer from London, wanted to meet in person but had travelled to South Africa for his father’s funeral. While there, he was robbed and defaulted on an expensive design contract. Judith stumped up £140,000 ($175,000) before realising that she had been scammed: “John” was the invention of a fraudster.

Last year online-dating swindles cost Britons a record £39m, according to the City of London Police. Six out of ten victims are women and two-thirds are between the ages of 40 and 69. Few of the fraudsters are identified, let alone caught, but most seem to be part of organised gangs in west Africa or eastern Europe, according to Neil Masters of Victim Support, a charity. Operations resembling call-centres conduct detailed background research, compose scripts and work several targets at a time. A fake emergency, often en route to an in-person rendezvous, is the most common way to solicit money. On average, just 30 days elapse between initial contact and first payment. Among those victims who go to the police, the average loss is £10,000.

Britons are particularly vulnerable to such honeytraps. In Europe, only the Swiss spend more per person on online matchmaking services, according to Leading Dating Sites, a market-research firm. The English language makes Britons accessible to con men around the world. They are fond of online shopping and banking, so making electronic payments to strangers is second nature. They may even be unusually susceptible to hard-luck stories: Britain is the top-ranked European country in the World Giving Index, a measure of generosity produced by the Charities Aid Foundation.

Victim Support and other charities are raising awareness of the problem. And, fearing damage to their business, dating sites are doing more to protect their members. Some parse their messages for suspiciously early declarations of love, or check if profile pictures have been swiped from other sites. That catches a few larcenous Lotharios. But nothing is as effective as what Mr Masters calls the “golden rule”: never give money to an online paramour, however charming.