The confession, posted on an infidelity website, read like a scene straight out of a 50s pulp detective novel. "Sleepless nights and days filled with worry and suspicion were quickly becoming the only existence I knew," a heartsick lover wrote of fears his girlfriend was fooling around. "For my own good, I needed the answers that were so carefully being hidden from me."

But that's where 32-year-old Todd Campbell of Olympia, Washington, and Sam Spade parted company. Though deeply suspicious, Campbell did not hire a private eye to follow his femme fatale into the night and return with incriminating evidence. Instead, junking cuckold tradition, Campbell shelled out $49.95 (£35) for the CheckMate infidelity test kit and caught her himself. "I have since enjoyed peace of mind that had been absent for so long," he gushes proudly.

Yes, but if surreptitiously testing your girlfriend's sheets and knickers for semen samples is your only alternative, isn't employing a Humphrey Bogart clone preferable? Then again, in these times cost is a factor, which may explain why the CheckMate kit is doing monster business in the US (1,000 kits sold a month) and has now arrived in the UK for the bargain price of £59.99. The home test kit, which includes pipettes and swabs, is available for the first time across Europe at www.getcheckmate.info Since its launch last week, just in time for the Christmas party season, the website has averaged 400 inquiries per day. Is it just what anxiety-ridden British couples have been waiting for? Would recent public revelations of infidelity from the likes of footballer Ally McCoist, new Scottish first minister Jack McConnell and actress Amanda Holden have been concluded differently had these kits been available?

"It's the perfect solution for men and women who want anonymity and don't have resources for an investigator," insists Brad Holmes, marketing director of CheckMate, based in Seattle. "But what we find very interesting is that 85% of our clients in the US are men. The stereotype of the dutiful wife married to the cheating husband is outdated," he says. "If anything, now it's the other way round. Women (who statistics have long maintained stray less than men) are cheating like crazy."

Ah, the joys of liberation. Equality has, Holmes confirms, stretched all the way to infidelity. Illicit sex is no longer the province of the travelling businessman, but the Concorde-taking businesswoman too. "It's because of the rapid shift of the role of women in our culture," says Holmes. "Today, 50% of the workforce is female, so their proximity to men who are not their husbands is incredible. The women who once stayed at home, now have all these opportunities. It's only natural their men are worried."

Worried enough to sneak around after their wives and girlfriends and apply drops of an enzymatic sensitive fluid to the crotch of their knickers (women will secrete traces of semen for up to 72 hours after sex) or the car seat, or their sheets, looking for evidence of a tryst. Women can check on their libidinous husbands - his underwear, sheets, his office chair - using the same method. If semen is present, blotting paper rubbed over the surface of the fabric will turn purple instantly.

Holmes says nearly half of his customers have been married 15 years or more, and 40% are over 40, suggesting that there are plenty of women escaping stagnant marriages by finding satisfaction elsewhere.

Yet despite the popularity of these home test kits, traditional methods of uncovering infidelity are hardly taking a beating. Former NYPD detective, Jerry Palace, who runs the Check-A-Mate Investigations service in New York City, says business is stronger than ever, "because people want hard information. Just because you find semen, it doesn't tell you the identity of the man. And many husbands want to know who it is."

Which is where Palace comes in to the picture. Unlike the predominance of male users of infidelity kits, Palace's client list is 60% female and 40% male. For $200 an hour he can run video and audio surveillance on your cheating spouse, while for $3,000 he will arrange for a "decoy", a beautiful woman (often former police officers) to meet your man in a place he frequents and flirt with him. The decoy, who is wired, as part of her patter, will ask him a string of questions about his relationship with you.

"You would not believe what we hear. Some don't admit to being in a relationship at all. Others say their wives are dead when they are very much alive. It's amazing what men say when they are on the prowl." Palace says that when men or women come to him with suspicions, nine times out of 10 they are "right on the money. "Sometimes they note a change in behaviour, other times the mistress tips the wife off anonymously, hoping to drive a wedge into the marriage."

Once infidelity kit users find evidence of cheating, or when Palace presents it to his clients, the inevitable result is confrontation followed by some heavy-duty decision making.

What is interesting, says Holmes, is that when men cheat, women usually bolt. But when women stray, men often try to repair the relationship. "We get calls from men telling us they discovered an affair, but think it might be their fault. They tell us they want to go to counselling to make it work again.

"But women, they find out their man is cheating, and that's it. They want out. I think they feel so powerless in a bad relationship, and getting the evidence finally empowers them to move on."

But does even a reasonable suspicion justify treating your other half like a child, snooping in their knicker drawer instead of having a mature discussion?

Relationship psychologist Dr Robert Butterworth says no. He insists that what is crucial to diffusing the situation is understanding why people cheat, not sneaking around assembling evidence that they do.

"The genders cheat for different reasons. Men cheat for sex. Women cheat for companionship. Women are looking to another man to give them the emotional support they are not getting at home. Instead of spending money on a kit, men should spend more time talking to their wives and girlfriends."

"In my experience, the cheating starts when people stop working on their relationship. You have to stay committed. I always tell my clients, if you married him or her, there was something there to begin with. And if you work at it, you can get it back."

BACK