Breast cancer screening saves the lives of two women for every one who receives potentially unnecessary treatment, research suggests.
Some cancers grow so slowly that a woman may die from another disease first while other cancers would cause no harm.
Experts are currently unable to distinguish between these less harmful cancers and some more aggressive types, meaning they are all routinely treated.
Now new research has found that for every case of overdiagnosis, two lives are saved as a result of the NHS breast cancer screening programme. The issue has been the subject of debate recently after other studies suggested the programme does little to save lives.
Last week, Danish experts cast doubt on the benefits of mammography, saying there were few differences in death rates between women who are screened and those who are not screened. The latest research was led by experts from the Wolfson Institute for Preventive Medicine at Barts and the London School of Medicine and Dentistry.
It focused on data from some 80,000 women from the age of 50 and looked at Sweden and England before and after the introduction of screening. It found 5.7 deaths from breast cancer were prevented for every 1,000 women screened over 20 years in England. The number of estimated cases of overdiagnosis was 2.3 per 1,000 women over the same period.
The authors, writing in the Journal of Medical Screening, said: "The benefit of mammographic screening in terms of lives saved is greater in absolute terms than the harm in terms of overdiagnosis. Between 2 and 2.5 lives are saved for every overdiagnosed case."
The researchers concluded the "benefits in terms of numbers of deaths prevented are around double the harms in terms of overdiagnosis. Analysis of both data-sets shows a substantial and significant reduction in breast cancer deaths in association with mammographic screening."
Lead author Stephen Duffy said: "This shows that the benefits of screening outweigh the harms. Unfortunately, we haven't yet got a flawless screening test, and some cases that are picked up wouldn't have needed treatment. But for every case like this, screening saves two women who would have otherwise died from breast cancer."
More than 45,000 women are diagnosed with breast cancer each year in the UK and more than 12,000 die from the disease. Women aged 50 to 70 are invited for NHS breast screening every three years across the UK. From 2012, this will be extended to include women aged 47 to 73.