Eleven years ago, following a routine mammogram, Barbara Gibbs, 67, was told that she had breast cancer and needed a lumpectomy followed by radiotherapy. What she didn't know until yesterday is that there is a chance her breast cancer may not have been life-threatening. In fact, there's a possibility that if it had been left it wouldn't have caused any health problems at all.

"No one ever mentioned that," she says. "What I was told was that I had a breast tumour, that it would grow, and that there was no question about it - I had to have surgery and treatment."

Yesterday a group of 23 health experts published a letter stating that the NHS breast cancer screening programme doesn't only identify women who have life-threatening breast cancer. It also identifies women who have breast cancer which, left alone, "might never appear in a woman's natural lifespan". And the problem is, no one - not even the world's top oncologist - can tell the difference.

What is more, according to research published today in the British Medical Journal, for every 2,000 women screened regularly for 10 years for breast cancer, while one will have her life saved, another 10 will be treated unnecessarily - having all or part of a breast removed - as well as perhaps undergoing chemotherapy or radiotherapy. A further 200 healthy women will have a false alarm. The signatories to the letter suggest that women need to be better informed about such findings, "in order to make their own choice about whether to attend for screening".

It's all news to Gibbs - but does it make a difference? "It's interesting, I can see that," she says. "But when you're told you've got breast cancer it's a terrible shock and what you feel is - get it out. I don't want a malignant tumour in my breast, thank you. When I was told I had cancer, I was all for doctors removing my whole breast - but they said no, you don't need the whole breast removing.

"If they'd said to me, 'Barbara, you've got cancer and it might kill you and it might not,' I wouldn't have done things any differently. I wouldn't want to go around thinking, there's this tumour inside me that's maybe life-threatening and maybe isn't. That would be a terrifying idea. I'd think that if there was even a chance it could kill me I'd rather not have it there at all."

The NHS breast cancer screening programme started in 1990 and invites women aged 50-70 to have a mammogram once every three years. It is currently being expanded so that, by 2012, all women of 47 and over will be included. Over the years since its inception, the number of cases of breast cancer it has identified has risen steadily - over the last decade it has more than doubled, to 14,100 in 2007/8. But while three-quarters of these were invasive cancers - the most dangerous type - around 20% were ductal carcinoma in situ (DCIS), more than half of which never become invasive.

According to the screening programme's chief critic, Michael Baum, emeritus professor of surgery at University College London (UCL), "Anyone who dares challenge [the programme] has a terrible time." In Baum's opinion, it should be reviewed in its "one-size-fits-all" rollout, and reshaped to focus on those most at risk: carrying on with the service without asking questions of it is, he says, "complacent and arrogant".

Baum's colleague Jayant Vaidya - another signatory to the letter, and a consultant surgeon specialising in breast cancer at UCL - says that there's no doubt that screening saves lives, but that women aren't being fully informed about the process. He says that not only DCIS, but other types of breast cancer too, can behave in unexpected ways, depending on the woman.

"There's a price that must be paid," he says, stressing that these are his personal views. "The fact is that if you look at cancer under a microscope you can tell it's cancer, but you can't tell if it's cancer that's going to be life-threatening, or that it's going to be life-threatening in this particular woman. Some cancers will regress, and others will grow so slowly they won't be a problem. But as a doctor, once I know a cancer is there it would be unethical to leave it behind." Postmortem examinations, he says, have found that up to a third of women have undiagnosed breast cancer, which suggests that the disease is more common than we think, but - since these women have generally died of other causes - perhaps not as lethal.

What's not made clear to women, Vaidya says, is that this overdiagnosis is an inevitability of the screening service. "When women are screened, they should be informed of the facts," he says. "That's all I'm saying - they should know the whole story. It doesn't make it easy, but it doesn't mean they shouldn't be told."

Baum and his like-minded colleagues have been blowing this particular whistle for a while now - so why hasn't the Breast Cancer Screening Service done anything about it? Its leaflet Breast Screening - The Facts does say that women might be called back for tests that eventually prove negative, but it doesn't address the central point that some cancers may not actually be life-threatening.

Julietta Patnick, director of the NHS Breast Screening Programme, says that the leaflets are currently under review, and an additional leaflet is being considered for women who are called back for further assessments. "The NHS Breast Screening Programme is committed to helping women make informed choices about their breast-screening invitation. Part of this is helping them assess the risks and the benefits of screening for breast cancer," she says, adding: "It is estimated that the programme saves 1,400 lives a year."

Tony Hall, professor of medical oncology at the Christie Hospital Trust in Manchester and director of the Breakthrough Breast Cancer research unit, says that, in his view, Baum and the other signatories are right to express their views, but wrong to do so through the press. "I think women probably should be given more information in the leaflet, and generally," he says. But he thinks it would be wrong to undermine the takeup rate for the screening programme. "The fact is that the death rate from breast cancer has gone down between 30 and 40% since screening was introduced. And while no one knows the exact reason for that - and while it's likely to include the use of the drug tamoxifen - it's very likely breast screening has played its part."

Hall also believes the figures quoted in the Baum letter are disputed by some doctors. "I understand that the more accepted figure is that for every 500 women screened over 10 years, one life is saved," he says.

Emma Pennery, clinical director of Breast Cancer Care, says the truth is that different patients need different levels of information. "Everyone should have access to as much information as they want, but different people do have different needs and can process different amounts," she says. "We'd definitely welcome a review of what women are told, and we also think women themselves should be consulted about how much they want to know.

"The truth of the screening programme is that once you go hunting for something, you will find it more. It's unavoidable that some people will have treatments they didn't need, but there are studies that are looking at which patients need which treatments, and that's an attempt to stop overtreatment."

Meanwhile there are women out there who have every reason to be grateful for taking the path of greater safety. Jenifer Duffield, 72, who lives in Farnham, Surrey, was told four years ago that she had DCIS and that, while doctors couldn't be sure whether it was life-threatening or not, the only sure treatment was a mastectomy. "I was very unsure about going ahead with it," she says, "but in the end I was persuaded it was better to be safe. After the operation, the surgeon said he'd found two stage-two tumours that hadn't shown up on the mammogram. Without surgery, I'd be lucky to still be here today."