Alex Thomas is rather different to many other stepmothers for one simple reason: she is prepared to confess to the extent of her feelings, or rather, the lack of them, towards her stepchildren.

As she will tell you, the best-kept secret of step-parenting is that just because you fall for your partner, it doesn't mean you'll take to their children. The truth is, you're more likely not to. One wonders why it is such a crime to admit to such a universal reality? We're not supposed to unconditionally love our partner's parents, after all, so why should their offspring be a different matter? As step-families are the fastest-rising family form we have, why is it so difficult to admit to the ambivalence so many of us experience daily?

"Do I love my stepchildren?" reflects Alex, 30, and a freelance radio presenter. "No, I don't. I don't feel the same intensity for them that I do for my partner, parents or even siblings. But I'm very fond of them. I want to be part of their developmental process and I enjoy their company but ..." she falters. "Love is still too heavy a word to use now."

Alex admitted as much very publicly when she was launching her website, www.childlessstepmums.co.uk, offering support to other stepmothers who fall in love with a man but not his children. It has already attracted more than 1,200 members, many logging on to confess to what would be completely taboo in any other context.

"Weekend from hell," moans one stepmother. "Am I terrible for being glad he's not allowed access to the kids?" reads another. Reassurance comes swiftly, "I would do anything not to have my stepdaughter over every other weekend," and "Ever so lucky. I'm green with envy! How did you manage it?!"

You sense a certain amount of glee at being able to express such forbidden feelings about their stepchildren or "skids" as they're not so lovingly referred to. The dilemmas are ones that usually remain hidden: the stepmother full of guilt because her young stepson told her he loves her and she was "struck dumb" because she "doesn't have those feelings". Another asks for advice on how to deal with a teenage stepdaughter who "can't even stand to hear my name being mentioned". Another confesses, "I'm worried because I hear so many of you love your kids and I, well, don't."

Bravely, or possibly naively, Alex hasn't been afraid to air such dark thoughts. One newspaper headline after the launch ran with her admission, "I wish my stepchildren had never been born". Yet three months later, she still doesn't regret her candour, modifying it only slightly. "My feelings have changed since then," she says. "But I'd still say, if I could have everything just the way I wanted, it would be me and Matt. If you take that to its literal conclusion, yes, I suppose you could say I wish they'd never been born. However, that's not the case - I do enjoy their company. They're intelligent, bright young people. But it is the case that I wish Matt and I could have got together before any of this."

Alex met her partner Matt, 43, over four years ago when they worked together on the same radio show. They became friends and slowly realised they had serious feelings for one another and, after much deliberation, Matt left his marriage. When Alex first began to see his children, Chloe, nine, and Tom, five, every weekend, she enjoyed her new role. Then one night, something shifted; it suddenly dawned on her just how excluded she really felt.

"One night I was lying by the fire and I looked up to see Matt on the sofa cosied up with Chloe and Tom either side of him. I felt really uncomfortable, totally on the outside. Normally, I'd cuddle up with Matt and now I saw something that was stopping me from doing that. He was giving his affection to someone else and, yes, I felt jealous, resentful, miffed. The fundamental conflict is, he's at his happiest when he's with me and the kids. I'm at my happiest when it's just the two of us."

Sometimes she'd try to embrace the new "mothering" role but much of the time Alex felt it "just wasn't me". There was the first camping holiday when she realised how intense parenting could be; the exhaustion and continual demands. Then the kids' unwitting mentions of shared moments with their mum; that holiday in France, the quality of her cooking as Alex served up a family lasagne. Sometimes, she wishes, she could just be left alone. Is she sure she's not just moaning about mothering in general?

"That's what my friends with kids say. Parenting takes an enormous amount of hard work, so does step-parenting - but the difference is that step-parents are doing all these basic practicalities and it's not through love."

As brave as it may be to say such things, didn't she worry about what her stepchildren would think? "Yes, Matt and I did discuss what we'd do if they found out about the interviews - and I would certainly expect to discuss it with them when they're older but, as it turned out, they didn't find out."

The fact that those feelings are "out there" doesn't appear to concern her. Nor is she worried that it could appear to some that she put her feelings above her stepchildren's in being so brutally honest.

"I've been very clear on that," she says, a steelier tone replacing the personable, bubbly demeanour. "I knew Matt and the children were hurting, but they had so many avenues of support. I still had problems. Just because mine weren't as far up the scale as theirs doesn't mean they're not worthy or don't exist."

We are in the sitting room of their modern home in a village outside Reading, conspicuously free of child clutter save for one bedroom given over entirely to toys for when the children stay. On the mantelpiece behind her there is a small framed photograph of Chloe and Tom grinning, either side of their dad, arms entwined around him.

Alex's candour is appealing but at times heartbreaking - from a child's point of view. She relates a bleak moment when she was looking after Tom on her own. "He was upset and started to cry, saying, 'I want my mummy, not you.' Outwardly I comforted him and said his dad would be back soon," says Alex. "But inwardly I thought 'Up your bum, I don't want to be here either.'"

It is the casual indifference that can sound so hardhearted - no wonder most stepmothers wouldn't dare to admit as much. Yet Alex does just that in an attempt to explode the myth of the wicked stepmother, not conform to it. "You don't like to think of yourself as a bad person. I thought, 'Am I really the evil stepmother here, wishing these children away?' Now I think, 'No, I'm not.' We're all capable of some fairly shocking thoughts; it's how we resolve them that counts."

We learn from our fairy tales that there are few figures we should fear more than the evil stepmother. There she is terrorising the lives of poor innocents such as Snow White, Cinderella and Hansel and Gretel, deeply unmaternal and wilfully destructive. One of her many crimes is daring to put herself first, to selfishly desire to be the most beautiful in the kingdom. The message endures: to put your own feelings first and to feel ambivalent about your stepchildren is pretty unforgivable. Which could explain the wall of silence.

"I felt utterly isolated," says Alex. "There were counsellors for single fathers, single mothers, stepchildren; every element of a broken family, in fact, apart from stepmothers."

Many stepmothers can't contemplate seeking help for what they feel are such "bad" emotions. Another reason why the stepmother archetype endures is that it touches on some elemental truths about jealousy, resentment and the battle for the father's attention; note that it is almost always stepdaughters, not stepsons, in tales who fare worse at their grasping stepmothers' hands.

"Jealousy was always the main issue for me," admits Jo Ball, 36, a life coach and step-parent counsellor who lives with her partner, Neil, and their two stepchildren in Devon. "Jealousy of the other women [the children have different mothers] and particularly Neil's daughter. She'd run over and sit on his lap and he'd be stroking her hair. It was an 'I-want-to-be-there' feeling I experienced - a jealousy of his relationship and shared experience with her. Often jealousy is too painful to admit to, so it just festers in the background, which causes more problems; we know 50% of second relationships split up due to these sorts of issues."

Maybe we should be surprised that it isn't even higher; how can a relationship happily develop when a parent has to acknowledge their partner doesn't love their children?

Patricia, 48, and a teacher living in London, is matter of fact about her indifference. "I don't hold any deep feelings for my partner's son," she says. "But it took me a long time to tell my partner. I felt he was trying to push too fast for things to be rosy, for me and his son to be close, and I had to be honest with him. I think he's accepted my feelings but it's not easy for him knowing how I feel about someone he adores."

Like Patricia, Alex also felt compelled to tell her partner how she felt. "At first he couldn't understand why I didn't love them. It took a lot of talking to get to the roots of why we feel how we do. I said, 'I think they're great kids, but I'm not feeling this. I hope it comes in time.'"

Reassuringly, Janet Reibstein, psychology professor at Exeter University specialising in family relationships, believes this honest response is also the correct one. And that it's important partners do admit these feelings to one another, in order to resolve them.

"Yes, in a way it is the right way to feel. That expectation of immediate love and intimacy is too much, and if you get forced into it, on both sides there'll be resistance, which will continue to create problems."

Even now that we have 2.5 million stepchildren in the UK, we still expect the impossible: "Love only comes after years; you can have an enormous attraction at the start to a partner, or as a mother bond with your baby, but otherwise it isn't something that happens automatically," says Reibstein. "Categorising the emotions that develop in step-relations is something we haven't done as a society. We don't have direct analogies and that's part of the problem. Instead we talk about feeling - or not feeling - like a mother, or a bit like an aunt, a sister or a good friend; but it's none of those. It's a different and important relationship that needs to be thought through and understood."

Until we find a better way to fill this vacuum, there are less mainstream arenas such as Alex's website which, beyond the supportive whingeing, offers a more sobering insight into modern step-parenting. There are women pushed to the limit by hostile stepchildren and resentful mothers, who feel unable to confess to fathers, and all this compounded by maintenance. They feel they're not at fault; they simply fell in love with men who happened to have kids.

"I've felt tremendous sympathy for some of the stories I've read," says Alex. "Even now you still get so many women coming on and saying, 'Am I a bad person?' and I always reply, 'No, these are basic primal desires to want to be with your man but to also feel that something is getting in the way.' As civilised human beings we have to deal with that."

· Some names have been changed.