Breast cancer screening does more good than harm, with any over-treatment justified by the number of lives saved, a study of 80,000 women has concluded.
Mammograms can spot dangerous tumours, but might also detect lumps that are essentially harmless, exposing some women to undue anxiety and surgery.
This has led to a debate among experts about the benefits of breast screening.
But this study suggests screening saves the lives of two women for every one who may have unnecessary treatment.
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.
In England from 2012 screening will be extended to women aged from 47 to 73.
The study by experts from the Wolfson Institute of Preventive Medicine at Barts and the London School of Medicine and Dentistry appears in the Journal of Medical Screening.
It focused on data from 80,000 women from the age of 50, and looked at data from Sweden and England before and after the introduction of screening.
The research estimated that 5.7 breast cancer deaths were prevented for every 1,000 women screened over a 20-year period in England.
At the same time, 2.3 women per 1,000 were told they had a lump but it was not clear if it was an aggressive form of cancer that needed to be treated.
Put another way, for every 28 cases diagnosed, 2.5 lives were saved and one case was over-diagnosed.
The authors of this latest study say the benefits of breast screening are clear.
"The benefits in terms of numbers of deaths prevented are around double the harm in terms of over-diagnosis.
"Analysis shows a substantial and significant reduction in breast cancer deaths in association with mammographic screening," they said.
England's NHS screening programme has been rewriting its leaflet for patients after concerns it did not provide enough explanation for women about their choices.
A new version of the leaflet will be published by this summer.
Richard Winder, deputy director of NHS cancer screening programmes, said: "There is a risk of over-diagnosis, and possible subsequent over-treatment, associated with any screening programme.
"But this latest, independent study shows that the risk of over-diagnosis is very much lower than some other recent estimates have claimed, and that the benefits far outweigh the risks."
Sara Hiom of Cancer Research UK says she hoped the latest study would reassure women that screening was valuable.
"What we need to remember of course is that detecting cancers earlier generally means improved survival. And we know through trials and through research that breast screening can save lives," she said.
Emma Pennery, clinical director at Breast Cancer Care, said it was aware the ongoing debate over the effectiveness of screening could cause "confusion and anxiety for women".
"This robust study clearly reinforces that screening remains an effective option for detecting breast cancers," she said.
However, Jayant Vaidya, a breast cancer surgeon at University College London and the Whittington Hospital, said the study was based on calculations that were opaque.
"Women who go for breast cancer screening need to know that there's a good chance they could be diagnosed with a cancer which is not harmful and may never have bothered them," he said.