One in three women �diagnosed with breast cancer after routine screening may undergo surgery or other treatment which she does not need, according to a study.

One in three women diagnosed with breast cancer after routine screening may undergo surgery or other treatment which she does not need, according to a study.

It found that in countries which offered regular mammograms, 35% of patients who were told that they had breast cancer were "overdiagnosed", meaning that their cancer would not have caused any symptoms or lead to death.

Researchers at the Nordic Cochrane Centre in Denmark said that overdiagnosis "can only be harmful to those who experience it" as many patients are referred for unnecessary treatment.

Professor H Gilbert Welch, professor of medicine at the Dartmouth Institute for Health Policy and Clinical Research in Vermont, said that women should be given more information about the potential benefits and harms of screening before they undergo a mammogram. In an editorial published on bmj.com, he said: "Mammography is one of medicine's close calls' - a delicate balance between benefits and harms - where different people in the same situation might reasonably make different choices.

"Mammography undoubtedly helps some women but hurts others. No right answer exists, instead it is a personal choice."

But Emma Pennery, clinical director at Breast Cancer Care, said: "Until it is possible to accurately determine the progression of cancers found through mammograms, screening remains an effective option for detecting breast cancers as soon as possible.

"As this review acknowledges this could lead to overtreatment in a percentage of cases. However, without screening women would face the prospect of having to wait for a visible symptom of cancer, such as a lump, to become apparent before treatment could start. We know that early detection of breast cancer may lead to simpler, more effective outcomes and would urge women not to be put off breast screening by this report."

In Scotland there are six centres and 19 mobile units which offer routine screening for women aged 50 to 70. More than 166,000 women in Scotland were screened in 2006/07 and 1395 cases of breast cancer were diagnosed.

But the research appears to suggest that more than 450 of those women have been "overdiagnosed" and would not have needed treatment for the disease, either as the cancer was growing so slowly that patients would die of other causes first, or because it remains dormant or regresses.

Karsten Jorgensen and Peter Gotzsche, at the Nordic Cochrane Centre, examined trends in England, Wales, Canada, Australia, Sweden and Norway, seven years before screening was introduced and seven years after.

The researchers estimated the level of overdiagnosis as 35% for invasive breast cancer, but when carcinoma in situ, a pre-cancerous condition, was included, it rose to 52% on average and 57% in the UK.