Rebecca Reynolds used to find that boys talked to her breasts and not her face. She hated their size (36G), and, to make matters worse, they were causing her excruciating back pain. She longed for a smaller bust - and she is far from alone. According to research by Marks & Spencer and, more recently, bra manufacturer Bravissimo, breasts in Britain are getting bigger. Because of changes in diet and, it is thought, increased use of the pill, over the past 10 years the average bra size of the British woman has increased from a 34B to a buxom 36C or D; almost a third of British women now wear a D cup or larger.

But while the nation's boobs have increased in size, the desirability of the naturally big breast seems to be on the wane. The flat-chested look remains the fashion industry's favourite, and it is no coincidence that best-dressed lists are usually topped by the likes of Kate Moss or Gwyneth Paltrow; designer clothes hang better on the streamlined figure. When we do see bosomy women on the red carpet, their breasts, often delicately draped in wispy chiffon with no visible means of support, are frequently of the pneumatic, cosmetically enhanced variety. We have become so accustomed to seeing the strangely motionless, perfectly spherical and unfailingly pert fake bosom, that the sight of a real pair of big boobs in all their natural glory has become unfamiliar and, to some, unattractive.

Some women are prepared to take extreme action, seeking breast reductions either on the NHS or privately. While enlargements remain by some distance the more popular operation, surgeons are seeing many women who want reductions. Cosmetic surgeon Judy Evans of the Nuffield hospital in Plymouth says that since she began performing reductions in 1980, there has always been a high demand, but that due to recent NHS rationing, it is now "very hard" to be treated in the public sector.

To qualify for an NHS breast reduction, you must be able to prove that the weight and size of your breasts are causing you extreme physical discomfort or serious psychiatric problems. The weight is quantified: it must be deemed necessary by the surgeon to remove 500g of fat and tissue from each breast. The waiting time can be anything up to three years. "It's a shame that it's now so inaccessible," Evans says, "because it's the most worthwhile form of plastic surgery, and many people can't afford private treatment. [A breast reduction can cost around £5,000.] Some people are very desperate." She estimates that she currently performs around one breast reduction for every five enlargements.

There are numerous health problems associated with big breasts, such as back, shoulder and neck pain, as well as skin rashes under the breast. A 73-year-old patient of Evans had become so debilitated by her breasts that she could barely walk before surgery. Then there are matters of self-esteem and body image. "Some patients may feel so anxious and self-conscious that they are unable to enter into a relationship," says Evans. "There's no typical age group; patients are all at different stages in their lives. Some may feel they are incorrectly perceived as bimbos in the workplace, and find it impossible to be taken seriously by male colleagues. My youngest patient, a girl of just 14, was unable to cope with the sexual attention, which verged on abuse."

But it is not an operation to be undertaken lightly. There are big risks: infection, nipple loss, loss of feeling and sensitivity, non-symmetrical breasts or nipples, bad scarring, an inability to breastfeed. "It's tragic if it goes wrong," says Evans. "Patient selection is very important: they must be physically able to cope."

Patients must also be psychologically prepared, and preoperative counselling is an important part of the process. Phillip Hodson of the Association for Counselling and Psychotherapy, says: "It is important that you are doing it for yourself, and not solely to change other people's reactions to you. There is a kind of religion of thinness in our society, and a pressure to conform to some fashionable notion of desirability. Women who are considering this operation should be very sure that it's not just about that - after all, the fashion might change next year." However, Hodson feels that the decision to have a breast reduction is "not an irrational choice on the whole. Usually the motive is avoidance of pain more than anything else."

There are alternatives to surgery. Losing excess weight through diet and exercise can help to reduce breast size. A properly fitting bra will help to keep movement down, which in turn helps to minimise back pain and supports the ligaments in the chest, delaying sagging. The change in shape of the average British woman has been reflected by the bra industry: manufacturers from M&S to Agent Provocateur (as well as more established specialists such as Rigby and Peller, which caters for sizes from 30AA to 48J) now stock an extended range of larger sizes. There are further developments in the pipeline. Vogue magazine recently reported that the European Space Agency is working on a smart bra, which will monitor body functions, while M&S is working with Loughborough University on a "posture bra", which will redistribute breast weight across the body.

Next week, a Channel 4 documentary follows three young women who are considering breast reductions. One of them, Lindie, had a reduction at 16 (having been teased mercilessly by boys at school), but her breasts have since grown. Now in her late 20s, she is considering a second operation, which she will have to pay for.

While she is thinking about it, she visits Rigby and Peller and is fitted with a sturdily constructed (though not unattractive) bra that lifts and supports her ample 36F bosom very nicely. She decides that, actually, she won't bother with surgery after all. "I've learned to love my boobs," she says. "They're part of me."

· My Breasts Are Too Big! is on Channel 4 on Wednesday August 11 at 9pm.