The following correction was printed in the Guardian's Corrections and clarifications column, Saturday March 11 2006

Our assertion that Margaret McNamee, the mother of twins conceived from frozen eggs last year, had had her unfertilised eggs frozen for religious reasons was misinformed in the article below. Ms McNamee has asked us to point out that her reasons were purely personal and we are happy to do this.

Until last year, only one "ice baby"- conceived from a frozen egg as part of the IVF process - had been born in the UK. The past 12 months have seen another three: twins born in September 2005, and a fourth baby due to be born earlier this year. Egg freezing - once a near-impossible technique with a negligible success rate - is becoming a reality.

The Holy Grail of the world of assisted reproduction, egg freezing is the ultimate in fertility control for women. Originally pioneered as a technique for those about to undergo fertility-threatening chemotherapy, its universal potential - were there no moral objections - is extraordinary. Christy Jones, founder of one of the US's first egg freezing clinics, describes it as "on a par with the introduction of the pill". If it were ever to catch on, it would in fact be far bigger than that.

Egg freezing allows women to choose a conception date by freezing eggs in their 20s for use as late as their 50s. It is fertility intervention for women who don't necessarily have fertility problems, a sort of precautionary IVF. And, because a healthy woman can carry a baby in her womb long after menopause, in theory it gives her (almost) the same reproductive window as a man.

The procedure is the same as IVF. The ovaries are stimulated to produce a batch of eggs, which are then removed with a needle under a mild sedative or general anaesthetic. The eggs are then drained of fluid, injected with a sort of antifreeze, frozen and stored at -196C in liquid nitrogen. They can be stored indefinitely.

Until last year, the technique remained relatively obscure due to its low success rates. But in June 2005, Dr Eleonora Porcu, Europe's leading authority on egg freezing, announced amazing new figures: 80% of eggs were surviving the freeze-thaw process in trials, and pregnancy rates were up to 20%, close to the success rate of regular IVF. So far, around 150 frozen-egg babies have been born worldwide, to patients who needed the technique for medical or ethical reasons - either because they had their eggs harvested prior to chemotherapy or because they had moral objections to the freezing of embryos.

Earlier this year, the London-based Science Media Centre organised a briefing session with a panel of fertility experts which concluded that "putting fertility on ice" could become commonplace in the UK within 10 years. New techniques, it concluded, mean that women in their 20s and 30s would be able to store their eggs for future use as a matter of routine.

In the US, it is predicted that by 2007 we will see the results of "lifestyle egg freezing": the birth of the first ice baby born by choice to a career woman, using eggs she had harvested a few years back. Around two dozen US clinics offer the service for $5,000. Here in the UK, half a dozen clinics can help you out for around £2,000.

It is the commercial potential for egg freezing that makes it particularly contentious. Some supporters of the technique - including Porcu - are opposed to using it expressly to delay fertility. Porcu is keen to advertise its successes in medical and ethical cases, but counsels strongly against widespread use - especially for what she considers the "wrong" (ie, non-essential) reasons. Others - such as Dr Gillian Lockwood, of Midland Fertility Services in Birmingham, who assisted in the conception of all four UK ice babies (all "ethical" cases) - believe the technique should be open to anyone who wants it, without judging their reasons.

Many in the infertility business are simply resigned to the onward march of market forces. Geoffrey Trew, consultant in reproductive medicine and surgery at IVF Hammersmith, undertakes egg freezing for cancer and leukaemia patients, but draws the line at patients who want to pay their way for personal reasons. He thinks that when egg freezing is not the only medical option, it offers a false insurance policy: "If a patient is 31 or 32, it is far better to discuss smoking, obesity and diet, and to advise her that she is better off not freezing her eggs but thinking a bit more about her fertility."

For him, the most important recent advance in fertility treatment is not egg freezing but ovarian cryopreservation for cancer patients, where a sliver of the ovaries is frozen and then reimplanted after cancer treatment. (Kylie Minogue is rumoured to have undergone this procedure.) The first such birth was reported in Belgium in October 2004. Perhaps predictably, though, barely had ovarian freezing been registered as an option for cancer patients than it was being offered as a lifestyle choice in several US clinics. The point is, Trew adds, patients are being allowed to pick and choose fertility treatments, provided they have the money: "The pressure is for the industry to comply with the patients' wishes - if they're willing to pay enough."

Caught in the middle of all this controversy are the parents of three-year-old Emily Perry, Britain's first ice baby. Helen and Lee Perry, from Ludlow, Shropshire, are reluctant adverts for the process. They know that without egg freezing they would be childless, but they emphasise that they did not set out to use the technique. They ended up taking the egg freezing route not as a result of their willingness to experiment with reproductive technology, but because of their reluctance. Both are Jehovah's Witnesses who believe life begins at the moment of conception. The freezing of embryos - routine in IVF, because they freeze and defrost more reliably than eggs - is unnatural to them, not so much because of their beliefs as Witnesses (it is not a policy of the faith), but because of their personal ethical beliefs.

Married since their late teens, Helen, 39, who has a part-time book-keeping job, and Lee, 40, who runs a building company, tried for a baby for six years before realising that IVF was the only way it was going to happen. Helen had blocked fallopian tubes following a burst appendix at the age of seven. As they researched the IVF possibilities, the realisation dawned: they did not want to participate in a process that creates multiple embryos with an uncertain future. They decided to ask for "natural IVF", where one egg at a time is fertilised and implanted. No extra embryos are stored or destroyed.

But on the day Helen's eggs were collected, she was diagnosed with hyperovarian stimulation: in one cycle she had produced 34 eggs. (Naturally women produce one egg a cycle. In IVF, fertility drugs are used to produce 12 to 15 eggs, of which the "best" are selected.) It wasn't safe to fertilise and implant an embryo. Their doctor suggested an experimental procedure: freezing the eggs and trying again a cycle later. On a second attempt, one egg was successfully thawed, fertilised and implanted. The rest remain on ice. They hope to conceive another ice baby, a sibling for Emily, at some point.

Much as they are grateful to egg freezing, they are uncomfortable about it becoming a lifestyle choice. "We wouldn't come out and say people shouldn't do it," says Lee, "but my view is you should have medical intervention only when it is required. As a bloke, I wouldn't want all that happening to me unless I had no choice. And it costs thousands - it's not like it's £50. You have to be committed. With the success rate at 20% or lower, it's a lot of time and money for something that there are fairly low chances on. You wouldn't put money on a horse at those odds."

Helen believes egg freezing will increase - not for lifestyle reasons, but because many people do not like the idea of "spare" embryos created by IVF (a process 30,000 women go through annually in the UK). She says people are more aware of ethics than we imagine, regardless of their faith: "I've met lots of women [who have had IVF] who aren't religious but are worried about these embryos that are left over and having to be discarded. It does bother them. They are brothers and sisters to their children, and that's difficult for them to cope with." In Catholic Italy this problem has been solved by banning embryo freezing; egg freezing is now routine in IVF cycles.

The parents of last year's "ice twins" also had religious reasons. In September 2005, Margaret McNamee, 36, a teacher, and Michael Fahey, 39, an electrician, from Sutton Coldfield, West Midlands, had Isabella and Anna. (No one knows why but, statistically, frozen egg babies are more likely to be girls.) McNamee already had one IVF baby, Matthew, two. At the time of her son's conception, she had three eggs left over, which were frozen before fertilisation, because of her views as a Catholic: "We simply couldn't countenance freezing embryos because we would have seen that as freezing little people," she says. "This was a way of trying to have our family while staying true to our principles." The fourth baby, due in early 2006, was also conceived with ethical concerns in mind.

Dr Porcu, speaking from her clinic in Bologna, Italy, is adamant that ethical and medical reasons should be at the heart of decisions to use egg freezing. The technology, she says, was not designed as a form of DIY reproductive delay. She argues the very idea is anti-women because it allows society to maintain the pretence that having a family is a hindrance to a career. She argues that it is taking birth control too far: "You have to take pills. You have to induce superovulation. All this, not because you have a disease but for some hypothetical pregnancy after the age of 40? I think it is risky. And the idea of postponing a pregnancy because it is not accepted in your workplace when you are 30? This is something really violent towards women."

Dr Lockwood at Midland Fertility Services argues the opposite. Egg freezing is, as she puts it, "the ultimate in family planning". At her clinic in Walsall, on the outskirts of Birmingham, the atmosphere is cheery and unclinical. Lockwood has treated hundreds of IVF couples and dozens of candidates for egg freezing, a significant minority for "lifestyle reasons" (who pay their own way: the NHS funds egg freezing only for cancer patients). Down the corridor from her desk is an unassuming room: nondescript blue carpet, strip lighting. This is the cryostore. "In here there are probably about 10,000 potential people," Lockwood smiles, pointing at two dozen canisters half the size of beer barrels. Several contain hundreds of frozen eggs.

Lockwood, 50, is the sort of physician you trust immediately: bright, thoughtful, sensitive, an easy manner. She believes women should have access to technology to help them combine career and motherhood. She decided to specialise in fertility issues early: "When I was in my 20s, a consultant told me, 'My dear, you will have to choose whether you want to be a mother or a doctor.' I thought, 'We'll see about that.'" A mother of three, she had her first child at 26. You are inclined to believe her when she talks about how easy it would be to get your eggs frozen - and the benefits for women who currently have no chance of having a child in addition to the adult life and career they want. She is passionate about the freedom this would give women.

Lockwood is breezy about the procedure being undertaken for lifestyle reasons. Egg freezing, she says, involves around five visits to the clinic, plus almost two weeks of self-injection: "IVF has become as outpatient-friendly as we can make it. It's no big deal compared with a bikini wax." At one point during our conversation, she disappears to perform an egg-harvesting procedure on a patient. She leaves at 12.20pm and returns at 12.43pm. There can be side-effects - bloating, tiredness or nausea. "You might not feel your little black dress sits as comfortably as normal," says Lockwood.

Privately, the process at Midland Fertility Services costs £2,000 for the egg retrieval, with a £100 annual storage charge. If you were to return to go through the IVF process, have the eggs fertilised and implanted, this would cost another £800.

Lockwood says that, in her experience, the women who choose to freeze their eggs for social reasons are more likely to be in their late 30s or early 40s. "Often they've been in a relationship that they assumed was going to lead to marriage and motherhood - possibly for 10 years. Then at 37, 38, the boyfriend says, 'I don't think fatherhood is for me.' Or he meets someone else. This woman - who has always assumed that eventually a baby or two would come along - finds herself single with her biological clock running down quite fast."

Lockwood becomes quite emotional when talking about this. She has obviously been faced with the grief of dozens of women in this situation. She has to tell some of these potential patients that it really is too late for them: "I need to know that they know that by the time they're in their late 30s the chance of any of those eggs turning into a live birth is very low." Lockwood consents to freeze their eggs only if she is convinced that "they know this is not an insurance policy, that it is just a possibility".

The clinic has so far had few approaches from women in their late 20s and early 30s. Lockwood regrets this: "If only they knew that, from the age of 35, it takes twice as long to get pregnant as under 35. That from the age of 40, the miscarriage rate is 40%, and from the age of 45, the miscarriage rate is 70%." Whatever your age or situation, she says, egg freezing can buy you time: "Eggs can be stored indefinitely. It's not like fish fingers in your freezer where you need to eat them within three months."

One of Lockwood's "lifestyle" egg freezing patients, "Lucy" (not her real name) agrees - with some reluctance - to talk anonymously over the phone. Not, she says, because she is ashamed of what she has done, but because she is a "private person". Lucy, 40, a worker in the IT industry in the Midlands, is one of an estimated 100 British women who have had their eggs frozen because they eventually want a child but are not in a relationship.

Her scenario is straight out of Dr Lockwood's casebook. At 28 she met the man with whom she thought she would have children. They never quite got round to it, and when she was 35 the relationship broke up unexpectedly. She decided to have her eggs frozen two years ago, at 38, after seeing television coverage of the Perry family. Now 12 of Lucy's eggs are stored at the Midland clinic. She admits that she does not know if they will ever be fertilised. "There is no guarantee I am going to meet somebody," she says, "I just think that if there are options available that can put you in a position where you have a choice, I'm all for it. I wanted the pressure taken off me because time was running out."

The procedure was harmless, she says, although her views on it seem mixed: "I can't even remember it now. It must be like childbirth - you forget about it as soon as you've got your baby. People are scared of the pain of giving birth, but it's something you just do. You put yourself in that position because the end result is what you want. I remember being at home for a couple of days, feeling uncomfortable and quite weak. But it was nothing, really."

The likelihood of Lucy's efforts resulting in a live birth is small: "I can't remember the percentage they gave me, but I know it wasn't very high. It didn't bother me because I had no other choice." There are other issues, questions she can't answer. Will she meet a man within the 10-year storage limit? (Although she could apply for an extension past the age of 48, and even give birth after the menopause.) If not, does she go down the sperm donor route? Until what age is she prepared to wait? She just doesn't know.

In the meantime, it seems egg freezing is being recommended by health professionals in the US as an insurance policy for women in their mid-20s. Kristina Cashion, 27, a law student from Houston, Texas, popped up on one of hundreds of US internet fertility forums. We spoke over the phone. She went for a routine check-up at her doctor's last year: "As I'm lying back on the exam table, the nurse says, 'I want to talk to you about possibly harvesting some of your eggs and getting them frozen. Because with the career you've chosen, it could be many years before you have time to have a child and your fertility will cut in half by the time you're 30.'"

Cashion, who is getting married this year and plans a family eventually, was horrified. Worse, one of her law professors told her the nurse's concerns were legitimate and that she, too, wanted to let her students know they "cannot put off having children indefinitely". At a later check-up, another doctor reassured Cashion it was "nothing to be concerned about at this point in life". Still, it has become a hot topic on campus. Cashion says half her friends have investigated the cost of egg freezing (although she doesn't know anyone who has gone through with it).

So how prevalent is "lifestyle egg freezing" worldwide? There are no statistics, only estimates. Of the half-dozen UK clinics that would confirm they offer the service, four said they receive more inquiries from the media than from potential patients. But according to the January discussion on the future of fertility freezing at the Science Media Centre, that will soon change. Dr Simon Fishel, director of the Care Fertility centre at the Park hospital in Nottingham, concluded at the briefing: "What's going to happen - and we're going to make it happen - is that a lot of people will start using IVF who don't have a fertility problem. It will take a few years to come about, but that paradigm shift will happen."

On the US website of Extend Fertility, thirtysomethings with model looks queue up to proclaim their relief at having their "best" eggs in storage. Megan, 36, acupuncturist, Seattle: "It is an empowering process." Alexandra, 34, sales executive, Boston: "If you're over 33 and involved in a fast-track career, you must consider this now. You will never regret having frozen eggs, but you could seriously regret not having preserved them when you had the chance." Amanda, 39, finance executive, San Francisco: "Doing this gave me the courage to wait for true love."

Christy Jones, 35, of Boston's Extend Fertility, became her own first client when she had her eggs frozen two years ago: "I imagine we will see the first baby born as a result of elective egg freezing sometime in the next two years." But this is a spurious prediction. Despite having had had more than 1,000 inquiries since June 2004, Extend Fertility has frozen the eggs of around 120 women, with 40% of these classified as "elective" or lifestyle egg freezers. That's 48 candidates - hardly a revolution. (If the other US clinics have a similar take-up, we are talking about a few hundred women, compared with more than 4 million babies born in the US in 2004, according to the US census bureau.)

Dr Gillian Lockwood, however, has a powerful argument in favour of the technique. As IVF use increases anyway, surely it makes more sense to plan for what's really going on in women's lives, rather than bury our heads in the sand? "As a doctor, I would much rather try to help a 40-year-old get pregnant with her own eggs that she had frozen when she was 30. You have a statistically much better chance using a 30-year-old's eggs; a 40-year-old's eggs don't implant well... It's the age of the egg that determines the quality - it's nothing to do with the age of the uterus."

Dr Porcu, of course, disagrees. She finds this kind of family planning depressing: "To say when you are 30, 'Probably in 10 years I will need IVF so I'll freeze my eggs,'..." Her voice trails off. She is rendered speechless by the idea. "I don't think women should trust this. Not - and I want to be precise - because of the technique itself. It works. But to trust this technique for the planning of your reproductive life... Well, I am a bit perplexed by the idea." Suddenly she realises the one application it does have. She laughs. "Hmm, I think this is a way for doctors to earn money."