Researchers in Denmark believe women around the world are undergoing unnecessary treatment for breast cancer.
The team from the Nordic Cochrane Centre say one in three breast cancers detected by screening may be harmless and as a result do not need to be treated.
Research found some of the cancers detected are unlikely to kill or spread and others grow so slowly that the patient is more likely to die of other causes first, or the cancer remains dormant or regresses.
After analysing data from the UK, Canada, Australia, Sweden and Norway the team found cancer screening programmes could lead to "overdiagnosis".
The findings published in the British Medical Journal (BMJ) show about 37% of women aged 40 to 54 "who died from causes other than breast cancer, had lesions of invasive or non-invasive cancer at autopsy".
But half of these lesions would have been detected by screening, leading to some being treated, they say.
The team concluded: "Screening for cancer may lead to earlier detection of lethal cancers but also detects harmless ones that will not cause death or symptoms.
"The detection of such cancers, which would not have been identified clinically in someone's remaining lifetime, is called overdiagnosis and can only be harmful to those who experience it."