Trial by DNA: it's not just Liz V Steve

In the next few days, the future of Liz Hurley's five-week-old son Damian will be determined by nothing more than a cotton bud and a flimsy piece of plastic.

Steve Bing's name may not be on the birth certificate, but a simple medical test is about to prove beyond doubt whether the film producer is in fact Damian's daddy. On the result rests a possible multimillion-pound settlement - as well as Liz Hurley's reputation.

Paternity testing is a boom industry in Britain - and not just for the rich and famous. Studies show that at least a tenth of British children do not have the father they think they do, and more and more couples are discovering the truth - not least as fathers challenge childmaintenance demands. As a result, dozens of private companies have sprung up to sell kits over the internet or via mail order, and anyone with a couple of hundred pounds can now discover a child's true biological identity - often with traumatic results.

For stars such as the Hurley-Bings, the test will be conducted by a court-approved lab working under a strict code of conduct. After Steve Bing demanded a paternity test to back his claim that Damian is not his, Liz Hurley agreed to submit her DNA, and that of her son, for laboratory tests. Within days, the three of them will submit a drop of blood or a swab of saliva in the privacy of a doctor's surgery.

Others simply turn to the internet to buy home-based kits with names such as "Dadcheck" or "Papacheck" whose results can seriously affect a child's emotional wellbeing. At least 20,000 paternity tests are now conducted each year in Britain - some sold by companies that stand accused of cashing in on family crises. The reputable companies organise tests through GPs. At University Diagnostics' labs in Teddington, demand for paternity testing has grown tenfold in as many years. James Walker runs the laboratory that conducts thousands of tests each year, often on a court's instructions. It costs £390 to test three people to a 99.99 per cent degree of certainty. The sample - usually blood or cells from inside the cheek - is dabbed on to a stain card and returned to the lab in a tamper-proof bag.

"Nowadays we can use any sort of body fluid that contains DNA, as well as hair," Walker explains. Samples are heated and treated to extract the DNA, before a cocktail of chemicals is added to copy the genetic code. Genetic material is dyed before being scanned by laser to produce a series of coloured bands.

"We look for which bands match between the child, the mother and the putative father," Walker says. "You get half of your DNA from your father and half from your mother. We run at least 10 tests, and for each we see how closely the bands match. We can be certain whether you are the father."

For anyone who undergoes a test, it can be frightening to learn the truth. Robert Ross (not his real name), 50, learned that he was not Sam's real father during a bitter courtroom divorce battle. "Suddenly my exwife stood up in court and announced that this seven-yearold was not my son," Robert, a computer engineer, recalls. "It was frightening - it made my mouth go dry. I was representing myself, so had to stay very unemotional, and the judge asked if I'd like to say anything. I said I wanted a DNA test, and was surprised when my ex-wife agreed." Sam also agreed to the test, so the three of them attended a registered clinic. Four weeks later, Robert received a letter explaining that he could not be the boy's father.

"It was a very unemotional letter, and it left me feeling absolutely forlorn," he says. "I went back to the court and asked to spend some time with my son, and we spent his birthday fishing. We sat on a rock by the sea and I asked him what it meant to him. 'You're still my dad,' he said. 'You're not getting out of it that easily.'"

Sam is now 12 and at boarding school, where he has been experiencing behavioural problems and was recently suspended. His mother has remarried and moved to Germany, although during school holidays Sam sees her and Robert on the Isle of Wight, where they both have homes. Sam has been shown a photograph of his biological father but has declined to meet him.

"I don't feel any differently towards Sam," Robert reflects. "In fact it's made me more protective towards him. He's got me, and knows he can trust me. I'm his dad, and that's it."

For Edward Kanauros, a computer salesman from Cambridge, a simple test bought over the internet was enough to change his life. Kanauros had always joked that his daughter Laura, now 21, looked nothing like him. But Laura stopped seeing the funny side during an argument with her mother last year, when she was suddenly told: "By the way, he's not your real father."

"When she was born, I had been suspicious that she wasn't my daughter - call it male intuition," says Edward. "I was now determined to find out the truth." Although he had divorced Laura's mother 15 years earlier, he had continued to pay £200 a month towards the girl's maintenance. He logged straight on to the internet and, without telling his ex-wife, bought a DNA testing kit.

"Laura and I had to send in photos, which the doctor signed to verify who we were. Two weeks after giving blood, we both got letters saying there was no way I could be her father." They were thrown into emotional turmoil. "I realised that my ex-wife had spent all these years lying to me and to the children. It was so cruel to them. But I couldn't let the lies continue - however painful the truth was."

Laura confronted her mother with the test results. She then went to see her biological father, who had been unaware of her existence. Edward, now 54, is trying to recover some of his maintenance payments. "I still love Laura, and though our relationship has never been very strong, she tells me I'll always be her dad," he says. "The truth can hurt, but it's better to know."

Robert Ross agrees that, for Liz Hurley and Steve Bing, any result will be better than not knowing. "I don't think Liz Hurley should be afraid of the test, though it could have been done away from the limelight, for the child's sake. You just know that as soon as he can speak, he's going to be asked what he thinks of X being his father. But all children need is a bit of love around them."