Research suggests that one in 20 children is the product of an extra-marital relationship. Do mothers have a duty to reveal the identity of the real fathers? David Cohen reports
What should a woman who finds herself pregnant by a man other than her husband or partner do about her predicament? Should she be honest and risk losing her partner at a time when she is most vulnerable? Or should she keep it a secret, perhaps never to volunteer the truth to either husband or child?

There are no official statistics on the incidence of non-paternity in the UK, but the evidence suggests that just such a dilemma is faced by more mothers than we would care to imagine. Steve Jones, Professor of Genetics at University College London, claims in his book The Language of the Genes (HarperCollins) that "in middle class society, about one birth in 20 is of this kind".

According to the Lancet, British medical students are taught that the non-paternity rate is much higher: 10-15 per cent. Alarmingly, at the top end of the spectrum, a famous study by Dr Elliott Philipp in 1972 revealed that on the basis of samples taken from 200 to 300 families in south-east England, 30 per cent of the children could not have been sired by their mother's husbands.

Ann Casement, a Jungian analyst, says that in her experience, such situations are quite common but that the sooner the mother is honest about what has happened, the better.

"The truth is there in a family, and to try and repress it never works," she says. "She will have to lie to the people closest to her for the rest of her life, and that is traumatic and will distort all her relationships. At some unconscious level, the other members of the family pick up on the lie.

"The husband might dismiss the child, even brutalise it without really knowing why. As for the child, what is most damaging is to discover the truth when both parents are dead and there is no one around to answer their questions."

But Geoff Scobie, senior lecturer in social psychology at Glasgow University, disagrees. "A mother who divulges the truth usually does so because she feels guilty and wants forgiveness," he says, "but the effect is merely to unburden the problem on to others. The only justification for telling the truth is this: will it help the innocent victims, who in this case are the father and child? Most of the time, the answer to that is 'negative'."

Denise Knowles, a Relate counsellor, takes a middle path, arguing that the issue is not clear cut. "I would want to explore the woman's fears of telling the truth and weigh those up against the consequences of continuing the deceit," she says. "She is the one with the secret and she must take responsibility for her actions. But the male partner has a responsibility, too. If he has doubts, he must voice them or else he colludes in the lie.

"Ultimately, choosing the right course of action is extremely complicated, like playing three-dimensional chess, but I would hope that the child's interests would be considered first. You need only look at the number of adopted children who seek out their natural parents to realise that children have a desperate need to know. It requires a very good reason not to tell them."

Sometimes parents continue the lie, even into the child's adulthood and despite the child being overwhelmingly suspicious. The advent of DNA testing by ICI in 1987 has allowed suspecting children to take matters into their own hands. For the cost of pounds 475, they can have conclusive proof as to whether the man they have always called dad really is their biological father after all.

A mother's story: 'I will never tell her the truth'

Nine years ago, Frances, a 34-year-old nurse, conceived her daughter, Alice, in a "single night of drunken extra-marital lust".

"I had just qualified as a nurse and there was this doctor I knew at our graduation party. My husband wasn't there and I ended up having a one-night stand. It was the first time I had been unfaithful to Joe and the next day I told him what had happened. Our marriage of five years was already disintegrating and he didn't say much. I thought the incident was over, but 13 weeks later the unthinkable happened - I discovered I was pregnant.

I vacillated between wanting to tell the real father that I was carrying his baby and deluding myself that it really was Joe's child. I would sit in the local park and write down the attributes of both men, trying to choose who would be the best father for the child, but there were no rational options. The biological father was aware I was pregnant, but he was married with kids and I thought he would deny paternity. Initially, I assumed Joe knew it wasn't his child - we hadn't had sex for a month after conception - but when he didn't say anything, nor did I. One day my friend made the connection that Joe wasn't the real father and I began to panic that others might, too. I changed my job, moved neighbourhoods and tried to put my past behind me.

I began having nightmares, always the same one in which I was trapped in a perspex box in a dark sky, trying to push out. At first the sides would mould around my hand, but then it was like I was smashing my hand against concrete and I could feel the pain searing up my arm. At the height of my panic, all the sides would collapse at once and as I fell, I'd wake in a cold sweat.

I went through the pregnancy on auto-pilot, distanced from the changes taking place inside me. After six months, Joe left me to go travelling, saying that he wasn't yet ready to be a father. That in effect was the end of our marriage. When Alice was born, I felt no bond, no love. I became paranoid that someone would take her away, as if because of what I'd done I didn't deserve her. I only started to have maternal feelings for her after nine months when it hit me that she was a person. Then Joe returned to the UK, very apologetic for leaving me. Although he had remarried, he asked for contact. If Alice had resembled her biological father, things might have been awkward, but she is the spitting image of me and he was taken by her charm.

Alice is now eight years old and believes that Joe is her father. I will never tell her the truth because it would traumatise her and threaten what has grown into a loving relationship. Joe has never indicated that he doubts his paternity, but I admire him because I think that deep down he knows.

Sometimes I feel guilty. Who is the real father? Is he the person who's there for the child or the one who shoots some sperm? You resolve these things on one level, but not on another. Keeping the secret is a burden I will carry for the rest of my life. I'm glad we're divorced. I don't think I could sustain the deception within a marriage."

A father's story: 'Ruby's not your daughter'

Michael, 58, an author and media personality, brought up two children, Ruby, now 30, and Peter, 28, with Sally, 54, to whom he was married for 17 years.

"The day Ruby was born, I remember going from my wife's hospital bed to the local and shouting my friend Damien a drink. I was enormously pleased to be a dad, especially as prior to our marriage I'd had a child with another woman who had been given up for adoption. Ruby was an engaging toddler, but two years later, when our son was born, I began to have difficulty relating to her. Whereas you could spot that Peter was my son from 100 yards, Ruby didn't look anything like me: I was dark, she was blond; I was rangy, she was chunky. Moreover, she behaved in ways I couldn't relate to. By the time she was five, she had become an anti-social child with a dogmatic streak and she seemed to carry ingrained values - like a casual attitude towards petty theft - that certainly weren't mine.

The thing was, Ruby looked like Damien. I had no reason to suspect that Sally had been unfaithful, but after agonising, I confronted her and asked her whether Ruby really was my child. I got an extremely angry rebuttal. "Why accuse someone else of being the father just because you can't relate to her?" she said. From then on, I put such thoughts out of my mind, but I still struggled to accept how a child of mine could be so foreign. I kept battling with her, trying to change her, using every tactic from gentle negotiation to coercion, but it was like pushing a rock endlessly uphill and we became more and more distant. I felt terribly guilty. I went for counselling to try and sort out why I loved my wife and son but could not love my daughter.

For reasons unconnected to Ruby, my marriage to Sally drifted apart and after 18 years together we separated. One day, five years after our divorce, when Ruby was 20, Sally invited me out for lunch and said: "There is something you should know, something I want to tell you." I could see it was important, but I thought perhaps she wanted to say that she still loved me.

"Ruby's not your daughter," she said. I was too shocked to speak. I got up and walked out. I felt so enraged by the betrayal of it all. Once I'd calmed down, I felt enormous relief. It was as if the great mystery of my life had suddenly been resolved.

Ruby asked me to carry on being her dad but I said "no". She and I have never spoken since. I feel sad for her in that she is the one who has suffered most. Had Sally admitted the truth when I had asked her, before all the damage was done between myself and Ruby, I am convinced I would have been a better father. Instead of trying to make her into something she wasn't, I might have understood her differences and accepted her for who she was.

For me, the story has a happy ending. Last year my natural daughter - the one who was adopted out at birth - tracked me down. I hadn't seen her in 32 years, but from the moment we clapped eyes on each other, the connection was electric. You cannot mistake us for anything other than father and daughter. What does that say about nature vs nurture?"

A child's story: 'Not knowing my real dad is indescribably painful, like being in mourning'

Anthony Savory was a war baby, born in 1943, eight years after his brother, Philip. Now aged 52, he is a freelance editor and researcher living in Suffolk.

"On Christmas day, 1945, my mother's husband returned from the war and was presented to me as my father. But my first real memory of him was two years later when, as a four-year-old, I trundled into my parents' bedroom to thank them for my presents and he shouted at me to get out. He was inappropriately aggressive and there was something in the way he said it that made me realise he wanted me out of his space altogether.

For the next 10 years, he vented his displeasure on me. At dinner, I was not allowed to speak. If I objected, he would lash out. Once he ruptured my ear; I ended up in hospital with permanent tinnitus. He would say, "All you're fit for is to sweep floors in the supermarket". I tried to please him, wondering why he was nice to my brother and why my mother never protected me. After a while I stopped wondering and just tried to get through each day. By the time I was 15, I was a complete mess.

I never doubted he was my father. My mother had always been so emphatic, calling him "your father". But when I was 27, he died, and for the first time I began to notice inconsistencies between his looks and personality and my own. One day, my mother showed me a photograph of a man who looked just like me and said: "This is your father." I was dumbfounded. I looked at her and at the photograph. Then she snatched it back and clammed up. I replayed that incident over and over in my mind for the next 20 years, but whenever I questioned her, she would deny the photograph's existence and insist that her husband was my father.

Then a few years ago I got hold of my father's war records and found out that he had been stationed in Northern Ireland, nowhere near where my mother claimed he had been. Given my birth date, it seemed extremely unlikely that he could be my father. I asked my mother and brother if they would provide blood samples for a DNA test. My mother threatened legal action, but my brother agreed. The tests were run last year and showed conclusively that we had different fathers.

The truth has set me free. For the first time in 52 years, I feel confident and positive about life. People say: "What you don't know won't hurt you." That's rubbish. Not telling me was a way of enslaving me, of leaving me to perpetually mistrust my instincts.

Now that I know who I'm not, finding my real father, alive or dead, has become the most fundamental issue of my life. My mother is 87 years old and refuses to help. It's difficult to accept that she is so vindictive as to be prepared to take her secret to her grave. Sometimes in my mind, when I match my attributes to my real father's and imagine what he must have been like, I feel I would have loved him very much. It's everyone's right to know who their father is. Not knowing is indescribably painful, like being consigned to a state of permanent anonymity and mourning."

Real names have been used only in the case of the Child's Story.