With blond pigtails flying, Peri runs in from the garden where she has been making mud pies in towelling pants. She would like a drink of milk, please, or nun-noo, as she calls it. She hops into her mum's lap, curls up her long legs and pulls up her mum's cardigan. Moving from left breast to right, she pulls and pokes, kicking her feet against the chair until she is comfortable, holding a breast with two hands. Sucking loudly and talking as she sucks, she eventually falls asleep, snoring. Her mother smiles contentedly and continues the conversation as if nothing had happened. Standing, Peri comes up to her waist. She will be three in August.

Peri's mother, Helen Butland, 36, from Wolvey, near Coventry, sees breastfeeding not just as a means of boosting a child's immune system, but also as a mothering tool. It's for feeding morning, evening and in-between times, if Peri asks for it, but it's also a glorified cuddle, for when she has a knock or scrape. Helen fed both her other daughters - Erika, now eight, and Frances, now five - up to the age of four, including during two pregnancies and periods after both births when she "tandem-fed" baby and toddler. There was a time when her middle daughter, Frances, then three, stopped feeding after baby Peri arrived, but then she started again: "She came over when I was feeding the baby and said, 'I used to have that - can I still have some?' So I said, of course you can. And after about half an hour of sucking, she was actually getting some again."

Helen is a full-time mother and nonexecutive director of the family business (husband Ian, 32, is an engineer), to which she will return once the children are all at school. Once a radical feminist, she is now deadly serious about the business of being maternal. Indeed, the expression "matronly bosom" could have been created for her. She has been lactating for eight years - "but I know other women who have been for much longer". Peri, she says, can feed as long as she likes - certainly up to her fourth birthday, just as her sisters did. Helen believes that babies should be allowed to be babies for as long as possible. She is only, she says, following the World Health Organisation (WHO) and Unicef guidelines, which state that breastfeeding should continue to two years minimum: "I do it in the knowledge of the long-term health benefits - protection against diabetes, heart disease, cancer." There is a benefit for her, too, she adds: breastfeeding mothers are less at risk from breast cancer.

She realises her choice is unusual - "I remember feeling shocked myself the first time I saw a woman feeding a child in outdoor shoes," she admits - but over time she decided it suited her and her children. She chooses not to mix with parents who don't subscribe to the same views: "Early on, I did seek out mother and toddler groups, but I found the disapproval from them quite a knock to me - not about breastfeeding per se, but about the way I do everything - sleeping with my babies, holding them all the time. Other mothers had rules about separation, cots and so on." Cots, she says, are cages.

Her view is that, after a certain age, babies will find their own way off the breast. Her philosophy after the age of two is "don't offer, don't refuse". Gradually, when the child is ready, they stop asking. She believes it has made her daughters more independent, because they have well and truly had their fill of being babies by the time they stop feeding. "When they're three or four, it's still so much part of their emotional needs. Ideally, the nursing relationship will continue until the baby outgrows the need. In their early years, a baby has a powerful need to be with their mother - it is only by fulfilling that need that it will ever go away. The risks of depriving them of that are evident in society today — alienation, diminished capacity for truly loving others. And it serves the marketplace that we grow up unhappy."

In a country where we have one of the lowest breastfeeding rates in the developed world, Helen's experience is rare. She is one of a largely invisible group of mothers who practise "extended breastfeeding" - defined as feeding a child after the age of 12 months. In the US, these mothers belong to associations such as the Militant Breastfeeding Cult and IncitefulMamas. They breastfeed exclusively to six months, continue breastfeeding beyond a year and hate dummies, cots and prams (breast, sling and mummy and daddy's bed are best). American militant breastfeeders refer to themselves as "global lactivists", and if their spouse is supportive (which he would have to be) he is described as a "lactivist husband".

In the UK, extended breastfeeders are less aggressive and don't form a cohesive group, but you only have to peek into the internet forums for breastfeeding support organisations to see that these women exist in their thousands. Breastfeeding statistics count only babies who are fed up to the age of nine months - when 13% of babies are still breastfed, according to the most recent Infant Feeding Survey - so beyond that age there is no record of how many women are continuing to feed their children. (This figure would mean there were around 70,000 babies still being breastfed at nine months — and 530,000 on formula.)

Certainly the stereotype of the baby-addicted, neurotic breastfeeder puts some women off continuing. As one mother of two toddlers puts it, "It's going to bite, it's going to talk, it's going to walk - no way." However, Rebecca Kennedy, 38, from Kingston, Surrey, couldn't be further from what she calls the "hairy-armpits, grow-your-own-lentils" image of extended breastfeeding. With a supermodel figure, immaculately coiffed and looking about 10 years younger than she is, she is a walking advert for long-term "nummies" - as her boys called it once they could ask for it. Mother of Archie, 10, Hector, seven, Rollo, four, and Sorrel, 15 months, she fed all her boys to the age of three (including the first two simultaneously) and intends to feed Sorrel for as long as she likes. A former tax consultant in the City (her husband works in banking), Rebecca gave up her career when her first child was born and threw herself into full-time motherhood. She yanks up her Juicy Couture tracksuit top to feed Sorrel (who can talk, walk and has teeth, as well as long, bobbed hair held back with a pink glitter clip), and looks pretty close to "the most natural thing in the world" - the smiley catchphrase of every extended breastfeeder.

The myths about droopy breasts, biting and never ovulating again are nonsense, says Rebecca. Your breasts are affected not by breastfeeding but by your experience of pregnancy. You don't feel the child's teeth. In terms of affecting fertility, it depends on the woman - some ovulate at five months if they're still feeding, others not until 15 months. Despite Rebecca's evangelism, though, she is discreet. "Most people don't know how long I fed my children because I don't go around advertising the fact. If they do know, they are very surprised about it." She has always avoided feeding her children in public after the age of two ("I would just distract them or say mummy's not wearing the right T-shirt") because of adverse reactions.

She admits that lactating for 10 years with only a few months' break during her pregnancies has been tough: "Breastfeeding is a bit like eating Mars bars - the first one is delicious, but then the more you have, you start to feel sick. I have mixed views about it finishing."

You don't have to eat more, as you do with a tiny baby, to maintain your milk supply, but ultimately it is tiring, especially around pregnancy. So why does she continue?

"There is always a nutritional benefit to breastfeeding," says Rebecca. "The real advantage is if they get a tummy bug or something, they can still feed." Aren't they overly dependent on her? "I think that, by allowing your child to be with you, you allow them to leave you when they're ready. We never had the terrible twos - if they were hungry or frustrated, they could feed."

There is no medical reason why a mother shouldn't feed her child as long as she - and the child - wants. Dr Miriam Stoppard, author of The Family Health Guide (Dorling Kindersley), says the health benefits to a baby continue no matter how long they are breastfed. "Breastmilk contains a hormone that helps the digestive system," says Stoppard. "In breastfed babies, the system is then mature enough to take mixed feeding [with solids] and robust enough to resist allergies. Breastmilk also contains some great mood-enhancing chemicals - opioids, which make the baby contented and sleep well. And breastfed babies definitely cry less."

Psychologist Dr Ros Bramwell of the University of Liverpool argues that it is only our societal attitudes that militate against later breastfeeding. "The breast is very sexualised and people's attitudes to seeing an exposed breast are very negative - even in private," she says. "The issue of 'who are my breasts for?' can cause a lot of conflict in women. Emphasising the benefits of breastfeeding is great, but it doesn't address the difficulty of this problem. I think with extended breastfeeding, it gets worse - there is a huge conflict between the sexuality of the breast and using it to feed. The unusualness is what makes it shocking." This is also connected to our perception of children and how quickly they should grow up, she adds. "Breastfeeding is seen as something that is only for babies - we see it as a babyish thing to do."

Lisa Cavadias, 30, a pensions administrator from Flitwick, Bedfordshire, says that, on the contrary, her breastfed daughter, Marie, two, is more advanced as a result. Marie learned the baby sign for breastfeeding (hand squeezing the fingers, as if milking a cow) at the age of one and now speaks in seven-word sentences, including: "Please can I have some mummy milk?" (When asked why she likes mummy milk, she replies, "Because it tastes nice.") A sunny, salt-of-the-earth type, Lisa is more like the traditional 1970s breastfeeder: long hair, flowing, flowery dress with breastfeeding flaps, literally barefoot and pregnant. She obviously enjoys her relationship with her daughter - who is unusually bright and lively, stomping through the house in yellow Tweenies wellies - but it has not always been easy. "Once they get past a year," says Lisa, "they feed in all sorts of different positions. And she is a nipple-twiddler, which can be very painful. Sometimes I just want to say, 'I'm not just a breast, there is actually a mummy in here, too.'"

Lisa has had to contend with disapproval and hassle (she expressed at work in the early days, so her daughter could still drink breastmilk during the day). "I try to put her off feeding in public, because you get looks and comments once they start to walk. There have been comments I was meant to overhear: 'That's disgusting'; 'She shouldn't be doing that in here.'" Despite being eight months pregnant, she has continued to feed because her daughter enjoys it. "We hope it will help with jealousy issues," says Lisa. "She'll know that she is still my baby, too."

Gaby Jeffs, 38, a video director from Herne Hill, south-east London, also faced opposition to her breastfeeding. Her son, now four, was fed to 13 months. Sitting at the kitchen table on her mum's lap, daughter Elizabeth, three, is still delving around in her mother's shirt looking for "boo-boo", although she gave up feeding a few months ago. Despite this, she is no more clingy than any other three-year-old, and within seconds is distracted, running off to fetch her own baby doll - whom her mother allows her to feed with a bottle, with reservations. ("That is quite extreme of me, isn't it?" says Gaby. "But you can't exactly let a child breastfeed a doll, so I had to give in.") "There were times when my parents were saying, 'This has gone on long enough.'" I got pleurisy and had serious antibiotics when Elizabeth was about 12 months, and you can imagine what everyone said: 'Oh, yes, now is the time to stop.' Also, Elizabeth has had problems with the enamel on her teeth - a gestational problem, it turns out - and the consultant at hospital said, 'Hmm, that's breastmilk for you.' In public, even now if I am carrying Elizabeth, she will put her hand down my top and I get raised eyebrows." As another glamorous militant breastfeeder (she and her mini-me daughter have huge, brown eyes and Louise Brooks bobs), she, too, hopes late feeding is losing its links with the "brown rice brigade".

Anecdotally, at least, there is a suggestion that their numbers could be on the increase. UK-based international lactation consultant Pamela Morrison "strongly suspects" there are a lot of secret late breastfeeders around. Late breastfeeding is like co-sleeping (having your child in your bed), she says - no one likes to admit that they do it. "There is extremely strong cultural pressure to wean an older baby and the pressure becomes more and more intense the older the child, more or less forcing mothers to 'closet-nurse'."

Valerie Goedkoop, 37, was one of these women. A doula and breastfeeding counsellor from Tunbridge Wells in Kent, who until recently ran Breast Friends, a support group, Valerie fed her first baby to eight months and the second to 19 months. "At the time, when I went past a year, I didn't tell anybody. I couldn't be bothered having the criticism. When someone makes a comment, it's often a negative one - which comes from ignorance or guilt. I think there are lots of secret breastfeeders out there. I think people do it in the evenings and don't tell anybody."

Numbers are also likely to be increasing because of birth trends. Nowadays better-educated, middle-class women are giving birth later in life - all factors linked to the uptake and continuation of breastfeeding. Morrison explains: "The baby with the greatest chance of being breastfed to a year appears to have a mother older than 30, who was educated past the age of 18 and lives in south-east England."

Abigail Thompson, 18, from Bolton, Lancashire, mother of Jake, 22 months, and Arabella, seven months, is a rare exception to these statistics. A full-time mum, her partner, Colin Doyle, 22, is the manager of a skip company. She left college at 16 to have her first baby and found out about breastfeeding by surfing the net. "I have never met a mum of my age breastfeeding. The information and support available to young mums is terrible. I had the frequent advice of 'give him a bottle of formula'."

Abigail was recently breastfeeding her daughter outside a cafe in the local precinct and was asked to "go and do that somewhere else". She regrets giving in to "the aggressive advertising of follow-on milk" with her son, and was determined not to let anyone stop her breastfeeding her daughter, whom she is now happy to feed up to the age of four. Everything she knows about breastfeeding comes from the internet — the only other breastfeeding mothers she knows are in cyberspace. She now expresses 5oz every two days for her son ("so at least he'll still get some benefits"), as she notices a direct comparison between her two children - Jake got colds as soon as he went on to formula; Arabella, who has been fed exclusively on breast milk, has never been ill.

Of course, the well-publicised crisis in the breastfeeding world is the small number of women who stick with breastfeeding in the first place, let alone to a year or beyond. In the first week, 69% of babies are breastfed. By the end of two weeks, that figure drops to 52%. After six weeks, it's 42%. At six months — the age at which all babies should, according to government guidelines, remain on breastmilk exclusively — only 21% are still at it. Despite the semi-closeted militant mothers, there is a huge stigma attached to feeding a baby beyond this age. The fact that we are shocked by anyone other than the tiniest of babies suckling a woman's breasts cannot help. The "bitty" sketches in the BBC2 comedy Little Britain, in which actor Geraldine James is seen feeding a fully grown David Walliams and squirting milk everywhere, represent a fear that persists - that breastfeeding is weird, freaky and embarrassing.

It is perhaps because of our bitty-phobic attitude that we lag behind every European country with the exception of Ireland and France. In Norway, for example, according to Rachel Myr, a lactation consultant and midwife at a hospital in Kristiansand, breastfeeding initiation rates are at 95% (compared with the UK's 69%), with 40% of babies still breastfed (along with solids) at one year. In the UK, the rate is less than 13%, and the figures don't even go up to a year. Myr says, "I have worked on a maternity ward for 16 years, and I can't recall ever having met a Norwegian woman who would willingly admit to not having a desire to breastfeed. We never ask them whether they 'want' to breastfeed. It is assumed they do. It would almost be like asking, 'And will you be doing your own perspiring, or have you decided to go with commercial sweat?'"

Longer breastfeeding is the average in developing countries (where five is a normal weaning age) and in certain US outposts. There is extreme and there is extreme. One US survey by breastfeeding guru Kathy Dettwyler suggested that the minimum age for stopping breastfeeding should be two and a half years, and the maximum seven years. Dettwyler's 2004 survey of 1,280 American children who had been breastfed beyond the age of three found that more than 200 children breastfed over the age of five (a few dozen fed to age eight or nine). In 2002, one Illinois mother was investigated by child welfare officials after she was filmed breastfeeding her eight-year-old son on the TV show Good Morning America. (Madonna declared at the time, "That's incest".) For even Britain's most militant breastfeeders, this is beyond the pale: most take the view that a child will self-wean by the age of four.

Morrison argues that our distaste for older breastfeeding is not a given - it's purely cultural and has existed only over the past 50 years: "In a society like we have in the UK, where most babies are breastfed for only a matter of days, weeks or months, we describe breastfeeding beyond one year as 'extended'. On the other hand, if we are thinking in global terms, where most young children continue to breastfeed for two or three years, we would describe most children weaning before that time as 'premature'."

Cultural resistance is slow to break down, however. Vicky Johns, 34, runs a Bosom Buddy network in Blandford Forum, Dorset. She fed her first child to 18 months, her second to the age of three. "Even at breastfeeding support groups, women don't tend to breastfeed the older ones. I was quite embarrassed about extended breastfeeding. It is something that is done in private. It's such a shame." She used to think that, with the breastfeeding awareness initiatives, attitudes were changing, but now she's not sure. "Now that my son has started school - he's four - and I'm back in the real world [with non-breastfeeding mothers], I don't think there has been a change at all. More people have started with breastfeeding, but they're not continuing beyond a few weeks."

The biggest obstacle to making longer-term breastfeeding seem at all normal is that living as these women do is not an option for many mothers. If you want your baby to be exclusively breastfed and you work, you need to spend time expressing milk in your workplace. By choosing to breastfeed your child - especially exclusively, if you want the child to have no other milk until after the age of one - you are effectively choosing to become a stay-at-home mother.

While women are bombarded with messages of the importance of continuing breastfeeding, the support system available to them is shaky: organisations such as the National Childbirth Trust (NCT), La Leche League and the Association of Breastfeeding Mothers are staffed largely by volunteers. The danger of bottle-bashing is a big problem: all the extended breastfeeders are keen to stress that what they do is their choice, and is not for everyone. Gaby Jeffs believes that if breastfeeding for any length of time was considered more normal, there could be an open discussion about the problems with formula versus breastfeeding: "We should be telling people not that breast is best, but that breast is the norm. And the bottle has risks. There is not much information about that tells you the truth - that with formula your baby will be more ill, will see the doctor more often, will have more chance of heart disease and childhood diabetes."

Meanwhile, back in Coventry, after half an hour three-year-old Peri slips off her mother's breast into a deep sleep. Helen Butland laughs when she thinks about her own previous attitudes to parenting. "I used to think full-time mothers were letting down the sisterhood." She herself never intended to breastfeed past three months, and here she is eight lactating years later. "I don't see what I'm doing as altruistic - I just want my children to be happy adults and to fulfil themselves in whatever way they want, and to be independent from me. I think that introducing premature separation is not going to guarantee those things the way this form of mothering does. I would like society to breastfeed like this because I believe it would be a better place — people would be emotionally fulfilled."

· For further information on breastfeeding, contact NCT's Breastfeeding Helpline, 0870 444 8707; La Leche League, 0845 120 2918; Association of Breastfeeding Mothers, 0870 401 7711.