Sarah Jessica Parker and Madonna are among those swapping Ashtanga for arabesques. Ballet can improve strength, posture and fitness - and anyone can do it, says Linda Watson-Brown

It is credited with giving Kylie the dreamiest of derrieres. It is alleged to have been the real reason behind Sarah Jessica Parker's amazing pregnancy legs, which held no truck with the usual compulsory plague of varicose veins and fluid. It is even being lauded as this month's favourite Madonna exercise fixation. Ballet is on the way back and moving into a new audience.

For grown women across the land, the notion of signing up for ballet classes may, until recently, have been pretty remote. Barely buried nightmares of Saturday morning sessions in cold church halls, dressed in a concoction of net pieces and tulle as a barking harridan shouted ferociously about posture should have resulted in lifetime abhorrence. And abstinence. But adult resilience combined with a seductive new exercise video may be about to change an entire generation's view of arabesques and pointe work.

The release of the New York City Ballet's second workout has added to the attraction. The video, which has shifted more than 250,000 copies worldwide, has just been released in Britain. Already the tasteful images of perfectly toned, graceful yet powerful bodies have been enough to make women desert their pilates and ashtanga for a session at the barre. The workout brings together aspects of ballet training and athleticism, emphasising muscle conditioning and definition, alongside flexibility, movement and balance. One-hour classes are fast selling out, and more than 50 different stretching and toning exercises can be involved in each session.

But why is this so attractive? "It's so different when you're an adult," claims 34-year-old Fran Webb from Glasgow. "A couple of years ago, when I was working in London, a friend introduced me to the first NYC Ballet video. I was hooked. As a little girl, you do get drawn into the fantasy of wanting to be graceful and beautiful - ballet seems to offer all of that. However, the reality when your mum drags you along never quite lives up to your expectations. I don't think it ever leaves you if you've been that sort of little girl. You get taller and heavier and clumsier - but you still want the spotlight on you as you perform the most beautiful Swan Lake ever!"

What aspiring thirtysomething Fonteyns are now discovering is that ballet offers a lot more than floaty skirts and carbohydrate denial. Images of Sex and the City celebs and perfectly choreographed music videos brimming with ballet-toned bods make it an aspirational way to exercise - but does it actually work?

"It is an incredibly healthy form of exercise for adults," says dance instructor Victoria Cooper. A ballet expert with health-club chain Holmes Place, Cooper has moved to Edinburgh from London to cope with demand. "Ballet improves muscular strength, posture, suppleness, balance and co-ordination," says Cooper. "I teach ballet fusion to lots of office workers, and after a day spent sitting at a desk, the health benefits are enormous. The class I've developed combines ballet techniques and conditioning sequences in a way that makes them safe and effective irrespective of whether you have any previous experience.

"One of the main things is that you walk differently once you know the ballet basics. You hold your head up, you straighten the spine. Even if nothing else happens, that can make a person look better and thinner immediately. Men aren't so keen - and they rarely stick at it - but women are addicted once they start. Yes, it's about lunges and squats, but it has such beauty too that it can change how you feel about exercise."

Gyms are claiming that the impact of the NYC Ballet video is substantial. The Holmes Place chain has scheduled 15 sessions a week at one London branch. The exercise requires a lot more stamina and strength than flitting about looking lovely - but trainers are well aware of the fantasy attraction of the genre, too. Long, lean muscles are indeed a desirable thing - but who can resist the lure of the ballerina?

But if you're putting on a leotard for the first time in decades, isn't there going to be a depressing discrepancy between what you think can be achieved and what you are actually capable of? "Not at all," assures Cooper. "Ballet provides a low-impact, conditioning workout. You'll see a difference in your posture, balance and co-ordination and you won't be all sweaty and strained. You won't be performing a solo in Covent Garden by the end of the week, but clients are usually amazed at what they can achieve in a short space of time."

Morag Dawes, the artistic director of Dance Base - Scotland's National Centre for Dance - agrees. "Ballet provides a perfect combination of exercise and glamour. All of the movements in ballet bring memories - you remember the films and the images and think of famous dancers. It's irresistible. But it's also very effective. When you perform an arabesque, the very act of lifting the leg up at that angle squeezes the gluteus maximus in a really hard, really effective way. The muscles are stretched and lengthened. Look at Sarah Jessica Parker - she's lean, she has long muscles, she is living proof that it works and it's a life-cycle type of exercise too.

"Muscles are like chewing gum - they can be made stronger by being made compact. Then they can be stretched out all over again to be more effective and stronger every time."

All of the explanations and descriptions seem perfectly sensible. In our exercise-savvy culture, the technicalities and intricacies of ballet seem easily surmountable if it is simply just something we want to do. "It is actually a lot more accessible than you might think," reassures Dayes. "Ballet has changed a lot. It's no longer all about fairies and princesses. Years ago, you would go to ballet and watch slight, ethereal women being lifted and dominated and pinned down by butch characters. There was always the suggestion that these wispy girls would fly away or break if not treated in the right way. But now audiences will see feisty, dominating women who use pointe work for power. It's sexy now rather than virginal and pure. That has filtered down to classes - and it draws people in.

"The NYC Ballet videos have brought a new audience in that it is very stylised, very attractive. Billy Elliot has helped too, and done so much for those of us who truly believe ballet to be all-encompassing. If you add to that a generation brought up on MTV, you can see why it is popular. I look at Christina Aguilera videos and I see the influence of ballet. I look at moves children make when they come in here and I see that they have already been influenced, sometimes without even knowing it. Ballet is carved in time. Once you know ballet, you can do any dance - and any exercise. It makes bodies stronger. It works."

Classical ballet has indeed gone mainstream, superseding yoga as the celebrity exercise of choice, banishing high-energy Eighties routines to the hinterland of exercise fashionability. "I have been amazed by the difference it has made to my body," says Fran Webb. "I do two classes a week, but I also practise at home. It's a bit like callanetics in some ways because you can just pop your leg up on the kitchen worktop and dotiny movements that make a difference. I have more energy, I'm tighter, and I love the way I feel afterwards. The overall health benefits are great - but unlike with yoga, I do feel you can pop in and out of it more easily, I don't have to change my life to do ballet - I don't feel guilty if I have a doughnut or swear. It doesn't claim to make me a higher power - but it does make me look fantastic and feel great. At the risk of offending Private Eye, ballet has to be the new yoga - and I can see myself doing it when I'm 60."

'New York City Ballet Workout 2' (Palm Pictures, £12.99)

'Ballet helps change your body shape'

* Ballet is suitable for everyone at beginner level - but it is vital to find a good teacher who can accommodate your needs. For classes in your area, call the Council for Dance, Education and Training on 09018 800 014.

* Ballet increases flexibility and lengthens limbs, using all muscle groups. It also improves core stability - the key to preventing back pain.

* Rather than being a quick fat-burner, ballet tones and strengthens muscles. "It helps change the shape of your body," says Maggie Paterson, ballet teacher at Pineapple Dance Studios in London.

* "You need strength to dance, so it's important that you eat well," says Chris Thomson, director of learning and access at The Place, the London-based centre for dance. "Lots of pasta, fruit and vegetables - ballet's about having a positive attitude to your body," adds Thomson.

Kate Haffenden