TOM WAS 15 when his mother told him that she could no longer cope with his aggression and was sending him to live with his father. Now she has written a book about their fraught relationship, a story of such painful intensity that tears poured down my face as I read it. No mother could fail to identify with her anguish and guilt, or her sense of failure.

Yet many will also condemn. Middle-class mothers, especially those with degrees in educational psychology, do not usually throw out their adolescent sons, however vilely they behave. With Letters to My Semi- Detached Son, Helen Braid (not her real name) has laid herself on the line or, as she puts it, on the chopping block. 'People will say I should have been stronger, that I should have kept this child, no matter what he did, simply because I am his mother. But it was a personality clash between us. I had also taken him away from a father he loved and got on with extremely well.'

In her book Ms Braid graphically describes the minutiae of a disintegrating marriage. With her son to care for, plus the chores, and a thesis to write, there were not enough hours in her day. Her scientist husband told her there were better ways of handling their child. 'He always took Tom's side,' comments Ms Braid. 'If Tom refused to help, it was because my demands were too heavy. If we quarrelled, it was because I had over-reacted.'

As the cracks in the marriage widened, there were shouting matches in which plates were hurled and the child cringed. When he was three his mother walked out, taking him with her to live in an uncomfortable flat in Bristol. Eventually she met the man who is now her second husband and by whom she has another son, aged 12. But there were to be no Happy Families.

'My son was angry from the moment I drove away. He turned his face against the change with a cold, passive determination. The hostile silence he's always adopted towards us is actually very aggressive. I've read a lot of books about splitting up and they all say the same: that children go on and on asking for their parents to get back together, no matter what dreadful things have happened within the marriage. It's been terrible, knowing all that.'

Did her own parents remain together? 'Yes, they did.' So she doesn't really know what it felt like for Tom? 'I'd see his suffering and not be able to do anything about it. I just could not break through.' According to his mother, he was an obstinate and critical child, always pulling her away from what she was absorbed in. 'In the end I would either lose my temper or, worse, give in to his demands with bad grace.'

One of Tom's weapons against her was the mess in his room. One day, when told to tidy it, he smeared blue face-paint everywhere instead, and she shouted the unforgivable words: 'I'm sick of you, sick of your miserable face. Jim and I would be happy if it wasn't for you. Why don't you go and live with your father and leave us alone?' When she got no reaction - another of Tom's weapons - she hit him and hurled him across the room.

'For both of us it was a watershed,' says Ms Braid. 'I had gone past the point of no return. He would not forget or forgive. The damage was done.'

As Tom reached adolescence he got into heavy metal, his clothes were torn and filthy, he didn't wash his hair. Other mothers told her that their tough teenage sons still had a soft spot for them. 'But from Tom there was no warmth, no contact. He said I was too strict. Friends advised me to ignore his room, but maybe they did not have in their houses a cesspit on the scale I did.

'Tom made me feel I was the only one who said 'no', I was the only fishwife saying: 'No, you won't go to the pub, you're only 14. No, you won't stay out all night, I won't allow it.' '

Tom refused to accompany the family on a holiday to California. They returned to chaos. 'There was a filthy sink, mounds of unwashed plates and cups. The vacuum cleaner and my hair dryer were broken. The larder was stripped bare, the cats starving.'

The real crunch came when Tom got into his mother's computer and read the book she was writing - this book. This time she did get a response. 'It's a load of crap, libel, rubbish,' he shouted. 'Your own one-sided version. It's me you're writing those lies about. It was so fucking disgusting, I pressed the button on it.' Says Ms Braid: 'He stole it from me. He had no right.'

At his father's, she says, Tom has total freedom. 'There are no rules and no criticism. I just can't imagine the person he is away from me. As far as I can tell he's all sweetness and light, but I cannot reconcile such behaviour with the difficult, violent child I know.'

Now 18, her son is taking a year off from studying and has gone abroad to work. Ms Braid will not say where. 'The publication of this book has been timed very carefully indeed. I want to keep the limelight off my son. I don't want him to be found by reporters wanting a sordid story. And it would be fairly sordid, I think, if they got hold of him. I am trying to protect my son.'

She loves Tom, but doesn't like him. 'Maybe he'll always have an angry relationship with me. My ex-husband has fed that anger, fanned the flames. As you can imagine, he's not pleased at the prospect of all this coming out. To him I am an appalling woman. But then, like many men, he's emotionally immature. They apply themselves to their work but put on blinkers towards the rest of their lives.'

For the time being, she says, so long as she knows Tom is all right and she remains in contact with him, she is happier with the situation this way. 'To salvage the family - my husband and other son - he had to go. The three of us were at the end of our tether. I was piggy in the middle. If he had stayed, I think it would have split the marriage. So there was really no alternative.

'And I think the decision was right for my son. I think he needed to go, to find himself, away from me. Perhaps in future he will look back and see it as the right decision. Maybe I should have done it earlier, before things got as bad as they did. It was traumatic for him, but it was right.'

Ms Braid began writing out of desperation at first, 'to see if I could make some sense of such an appalling unhappy mess'. Now she hopes her book will help other mothers who are at their wits' end. 'I felt so alone,' she says. 'I'd like them to know they aren't the only ones.'

In spite of her anguish, Ms Braid comes across in her book as a selfish mother. It is impossible to escape the feeling that it would have been better for her son if her raw expose of damaged lives had remained private. It seems a strange way of 'protecting' him. As he said, there are two sides to every story. This is his mother's. What, I wonder, is his?

'Letters to My Semi-Detached Son' by Helen Braid, Women's Press, pounds 5.99.