Gone are the days when adultery was so taboo that affairs generally happened by accident. Now increasing numbers of women set out to stray as if extramarital sex was just one more thing on their to-do list.

One of the challenges Sheila hadn’t expected was where to hide her sexy lingerie. ‘I went out and spent a fortune at Myla on gorgeous transparent bras and G-strings – things I’d stopped wearing for my husband, Peter, even before we were married.

‘The problem was where to keep them and how to wash them? I could have worn them every day but after 13 years of greying M&S pants and flesh-coloured bras Peter would have been rightly suspicious. I ended up stuffing all my new buys inside the legs of a manky old tracksuit. I washed them by hand, locked in the bathroom, and dried them with a hairdryer as I didn’t dare hang them up.’

Sheila, from Buckinghamshire, is 46, a doctor and mother of two teenagers, and for the past year and a half she has been having an affair.

Tall, strong-featured and dressed in a cashmere sweater and wool trousers, Sheila is the kind of woman you see in the aisles of Waitrose, the front row of the school carol service. But once every fortnight or so she tells Peter, 48, a company director, that she’s meeting a (well-briefed) girlfriend for dinner. Instead she goes to a motel room to see her lover, Michael, also 46, a medical sales rep whom she met at a conference.

‘We have sex, we chat, we go back to our families,’ Sheila tells me over a cappuccino. ‘I’m not looking for a soul mate, I’m looking for physical release. I stopped fancying Peter years ago. The girls were small and sex with him had long been just another chore, like loading the dishwasher. Finding out that I could still enjoy a physical relationship was …’ She searches for the word. ‘Mind-blowing. I thought that whole side of my life had died, but with Michael it’s been revived.’

From Wayne Rooney to Tiger Woods, barely a day seems to pass without male infidelity making the headlines. Female infidelity, however, has a far lower profile and, in popular culture at least, tends to have terrifying consequences. The two most famous adulteresses in fiction, Madame Bovary and Anna Karenina, came to sticky ends: one poisoned herself, the other threw herself under a train.

But today the guilt that tormented these classic heroines is largely missing. Thousands of women like Sheila are enjoying what they believe to be no-strings flings. Having witnessed the devastation divorce wreaked on their parents’ generation, they have no desire to end their marriages. Instead they are searching for variety in an otherwise humdrum routine.

‘I love Peter dearly,’ Sheila says. ‘He’s a good husband, and father. I like cooking with him and gossiping about the neighbours. He’s my pal and I’d never want to lose that. Sex with Michael is a purely separate thing; it’s about erotic abandonment, being seen as just a woman rather than as Peter’s wife, or “the doctor” or a mum. Any working mother will know what I mean. Every woman needs something that is hers alone. Some of my friends ride, some sing in choirs, I have Michael.’

The number of people having affairs is impossible to know, as few are truthful about their sex lives, but the recent Way We Are Now nationwide survey conducted by Relate showed that 34 per cent of women respondents admitted to being unfaithful, compared with 32 per cent of men.

Paula Hall, a sex and relationships counsellor and Relate spokeswoman, says that she expects to see a rising number of female clients who have been unfaithful, as the reasons for straying have changed. ‘Traditionally we believe a woman who has an affair is looking for emotional intimacy, whereas men are looking for sex. But that’s outdated.

‘Women feel more powerful and in control and as a result there are certainly more women being unfaithful because they fancied somebody and were away for a weekend with the girls or on business and seized an opportunity. Or it’s a case of a woman being neglected in the bedroom in the way a man once was and wanting to find a way to scratch her itch.’

For a wired generation, the automatic place to turn to for satisfaction is the internet. ‘They probably did online dating when they were single, so that’s where they’ll immediately head to see what’s out there,’ says David Miller, who runs Loving Links, an online dating service and one-to-one introduction agency for ‘nice, middle-class attached people’.

Since founding the business 15 years ago Miller has noted a huge increase in younger female clients. ‘It used to be very much an empty-nest phenomenon for the over-fifties, but this week alone I’ve met five new women all under 40. Women in their twenties and thirties belong to the “Because I’m Worth It” generation. They had a much freer sexual history when they were single and they have far more expectations of good sex.

Of course, once they’re married it doesn’t carry on being like in the magazines but their copy of Cosmo is still popping through the door every month shouting six new ways to heighten orgasm tonight and they think, “It’s not going to be with him indoors, he’s exhausted, but what a lovely idea …”‘

Once the seed is planted, outlets are everywhere. Most affairs start in the workplace, and a recent American study showed that women who travel for work are three times more likely to have had two or more concurrent sexual relationships in the past five years than women in general. But as well as seizing their chances, today many women and men plan their infidelities with chilling efficiency. They decide they want to commit adultery and set out to find a suitable partner.

Wendy, a 34-year-old full-time mother of two from East Sussex, was a ‘typical girl about town, with a few long-term boyfriends peppered with a handful of one-night stands on holiday or after a night clubbing’. Six years ago she married Billy, a stockbroker, gave up her administrative job and moved out of London.

‘I’d bought into a media image of rich husband, house in the country, stay-at-home yummy mummy,’ she says. ‘But it just wasn’t me. Billy is away a lot for work and I’m bored and lonely at home with the kids. I have mum friends but I missed the buzz of male company I used to have and I missed the irresponsibility of my old life – going out on a Saturday and not knowing who I might meet, where the evening might take me.’

To compensate for this, Wendy decided to take two annual week-long holidays alone. ‘The kids stay with their grandparents and I go to a spa or shopping somewhere like New York. I call it my “me time” and nobody argues. I do shop and have a lot of facials and massages but I also go out alone to bars in the evening and five times I’ve met a man.

‘Usually we just have a snog and a fumble but two have come back to my hotel room. It sounds dangerous, talking about it like that, but I trust my instincts to know who’s safe and who’s a serial killer. It’s about passion and spontaneity – having sex in the shower and up against a wall, all the things you see in movies that just never happen when you’re arguing over whose turn it is to wipe the kids’ bottoms.’

The advent of email and mobiles have made affairs easier to run than ever before in practical, if not emotional, terms. Yet suspicious husbands are also dab hands at checking their wives’ inboxes. All the women I spoke to were obsessive about covering their technical tracks.

Laura, 51, a reflexologist from Hertfordshire, with a teenage son, has had three affairs over the past 10 years with men she has met on various websites.

She uses specialised software to make sure her computer shuts down moments after she uses it and its history is wiped clean. She has two mobiles: one for general use and one for EMAs (extramarital affairs, to use the jargon), which can only be accessed by a pin number and is set on silent mode so that her husband, Brian, an events manager, can’t hear texts arriving. She checks at the same time every day before hiding it – separately from the sim card – in her Christmas-present drawer. ‘Then if Brian did find it I’d say I was going to give it to our cleaner,’ she explains, cradling her large glass of merlot.

‘You can’t be too careful,’ she continues. ‘You hear so many stories about people being caught out. One man I used to see had his wife discover us because he got a speeding ticket from Oxfordshire where we were meeting, rather than Birmingham where he said he was.

Another linked his mobile to his satnav when he was driving his family to his mother’s. A text came through and the satnav boomed, “Hi, sexy.” He managed to cause a diversion and got away with it but he almost crashed the car.’

Laura is adamant that her affairs are saving her marriage rather than putting it at risk. ‘Brian irritates me, like all my long-married friends are irritated by their husbands. He leaves the loo seat up, burps and expects his washing to be done as if by magic. He’s got a bit fat and resents any suggestion that he lose weight.

He’s never been the romantic type, never says, “I love you,” or tells me I’m looking good. My EMAs help me tolerate all that. I love the flirtations, the flattery. I’m looking to be adored, to be treated like a goddess much more than I’m looking for sex. It’s just so lovely to have someone compliment the necklace I’m wearing. My confidence has blossomed.’

But can a woman really have her ego bolstered, without losing her heart? Minna, 30, a part-time administrator from Glasgow, has had two affairs with fathers at the school her two young children attend, while her husband was working abroad.

‘The first time I did get hurt,’ she admits. ‘The man was married too but I deluded myself we’d run away together and when he backed off I was distraught – and to make it worse I couldn’t confide in anyone about what had happened. This time round I’m being much more businesslike. I tell myself it’s just a fantasy: a temporary release from the drudgery of my life, rather than a solution to problems that go very deep.’

Like most of the women I spoke to, Minna worried not so much about her husband learning of her affair as about what discovery would mean for their children. ‘He’s an adult but if they discovered this other side to me it would overturn their cosy little world,’ she says with a shudder.

And how would Minna feel if she were betrayed? She chews her lips. ‘I’d tell myself what goes around comes around,’ she says, but then adds. ‘No, I’m lying. I’d be very hurt. Illogical but true.’

So for thirtysomethings is monogamy now, as Miller puts it jokingly, ‘just a type of wooden furniture’? Certainly the temptations are ever increasing.

The therapist Andrew G Marshall, the author of How Can I Ever Trust You Again, cautions, ‘Both men’s and women’s sense of entitlement has gone upwards but sex is still incredibly difficult to talk about. Most people’s sex lives are probably a bit dull after 20 years; you want more and better but you don’t have the skills to go out there and negotiate them with your partner. It almost seems easier to go elsewhere.’

That was Minna’s experience. ‘My husband’s a sensitive, proud man. Saying “You don’t actually do it for me any more” would enrage him; he’d never sit down and talk about it rationally. It’s easier for me just to outsource my frustrations by finding men to sleep with just like it’s easier to hire a cleaner than have constant rows about keeping the place clean.’