'I WOULDN'T have had an affair if I hadn't felt neglected. David didn't seem to see me anymore - he never commented on my appearance, or even listened to the things I said. I felt as if I was constantly having to ask him questions, just to get a conversation rolling. In bed, our relationship was perfunctory.

"Then Mike caught my eye, one night, when we were having a drink after work. I'd known him for years, and he came over to me and said, very kindly, that I seemed rather down. I found myself telling him how unhappy I was, how I felt almost as if I'd become invisible in my relationship."

Sarah Jones is 36. She's been married to David, 37, for seven years. She's a legal secretary, he's an actor. They shared the housework and had what appeared to be a "perfect modern marriage", and until two years ago they were planning to start a family. Until, that is, Sarah began an affair with a work colleague, Mike.

"I'd known Mike since I'd started working at the law firm, and I'd always thought he was attractive, in a taller Tom Cruise type of way," she says. "He'd made it clear that he thought I was sexy, but I was married, I wasn't looking for anyone else. It boosted my ego, but I told myself I knew just how far to take it, and I thought he was a bit of a playboy and didn't take it too seriously. I hadn't realised what a good listener he could be, until that night."

Sarah, from London, says that what bowled her over was the fact that Mike actually listened to what she had to say and made her feel special, wanted, and desirable. The very qualities, relationship psychologist Susan Quilliam argues, which are the most likely reasons for women to begin an affair.

Extra-marital affairs have now become the major cause of divorce, and in 93 per cent of cases, according to solicitors Grant Thornton, which has an office in Newcastle, it is the woman who initiates the split. The first five years of marriage, the research reveals, is statistically almost divorce and affair-free, and if your marriage lasts over 20 years, then the vast majority will remain and both partners will be faithful.

But the greatest danger area for affairs is once the children are in their teens and more independent - statistically, when you have been married for between ten to 15 years.

Says Quilliam, who is also author of the Relate guide Staying Together: "Women are now sexual decision makers as never before. There are two big reasons - we have control over our own fertility, and we are financially independent. We don't have to "wait to be found" as women in the past might have been.

"Marriage used to exist to protect women - financially, for the most part - but now women don't need protecting in that way. The opportunity is there for them - they travel through business, they're away from the home for longer periods of time, and, put simply, they are far less likely to put up with an unsatisfactory relationship."

Women, however, are not rushing into affairs simply for sex. Sarah says: "I felt left behind in my relationship with David. His career seemed to be forging ahead, and I was the one with the more mundane job. He made me feel boring. Chatting to Mike that night in the wine bar made me realise that I can actually be fun. He was in his mid-thirties, really easy to talk to, and charming. I loved having Mike's total attention, and he made it clear he wanted me, in an extreme, passionate way.

"We started going out on our own, after work, and it was like being in a bubble. When I was with him, I couldn't think about David or anyone else. It sounds a clich, but I felt like a teenager again, butterflies in the stomach, obsessively thinking about him. Then he asked me to go and see a play with him. I said, 'That sounds like a date'. He looked at me, long and hard, and said, 'It is.'

"That night, getting ready - I told David I was going out with a girlfriend - I refused to admit what I was embarking on. After the play, we went back to his flat. The sex was incredible, I'd forgotten I could feel like that. From that moment on, I was plunged into a full-blown affair.

"I kept trying to justify it to myself, that David was neglecting me, that I wasn't somehow culpable. I kept telling myself that next week I would end it, but it was like a drug. I was totally intoxicated by him, and it's amazing how you can lead a double life, and somehow justify it to yourself."

But soon David found out about his wife's errant behaviour.

"It was the classic way - he found a text message on my phone from Mike," she says. "It detailed how he'd felt last time we made love - it must have been awful for David to read. When you're in the throes of an affair, you somehow convince yourself that your partner would not be so hurt, that they can't love you so much. On the contrary - David went berserk. He told me to get out, that he never wanted to see me again. He broke all kinds of things in the house, and it made me realise that he did love me, because he totally lost it. But he'd neglected me, he'd made me feel small - was I totally to blame? After several weeks we met to talk, but he said he could never trust me again."

While there may be more women having affairs, the reasons for doing so differ, according to Susan Quilliam.

"Men will be content to have an affair and sweep it under the carpet," she says, "whereas women become much more deeply involved. Affairs tend to become a replacement relationship - they are far more likely to think of spending the rest of their life with that man."

She feels that we, as a generation, put far more pressure on our marriages than ever before - perhaps unrealistically. "In the past, a woman was far more likely to live near her parents, she'd have a wide group of friends, and being at home, she'd have the time to talk to, and rely on, other people outside of the marriage," says Quilliam.

"Now, with both partners working, we seem to expect our husband to be not only our lover but also our best friend. We put everything into that one central relationship, and often it's too much."

She suggests that, in a way, we may be too dependent on our partner, to fulfil every need, largely as a result of the breakdown of the extended family. If that partner does not fulfil all those needs, then we're more likely to look outside of the marriage for someone who does.

Quilliam says there are four main reasons why women have affairs.

"I call them the four D's," she says. "Disappointment - she feels that the relationship doesn't reach her expectations and she wants more physical or emotional attention. Desire - wanting more, or better, sex. There is a direct link between the amount of sex in a marriage and its success. Disillusionment - feeling that their husband is not going to become the person they want them to be, and often women have affairs with men who are more successful at work than their husbands. Destruction - they use an affair as a way of levering themselves out of a marriage they already realise has broken down."

In my novel, the effects of the affair, between two halves of two couples who have been friends, is devastating and one pays the ultimate price. Sarah says: "I've been on my own now for two years. David won't speak to me at all, and, if I am honest, I still love him and I wish I had tried to talk to him about how I felt, before I began the affair with Mike.

"Three months after David left me, Mike went off with someone else, someone younger. I know that relationship didn't last, either. I let go of someone who was faithful to me, but who'd forgotten to show me that he loved me, in a way that mattered to me. We should have been much more honest with each other. Now I'm alone, I'd love to marry again and have children - but it may be too late."

l Playing With Fire by Diana Appleyard (Black Swan, £6.99)

l Names have been changed