Their conclusions deal a blow to the NHS Breast Screening Programme, the first such programme anywhere in the world, which screens women aged 50 to 64 every three years.
The Canadian review of mass screening found that the majority of "positive" screenings (tumours) later turn out to be false; that screening often leads to unnecessary tests and useless surgery; that negative screening does not rule out the presence of cancer, and the outcome for most women whose cancers are diagnosed by screening is unchanged.
About 5 per cent of mammograms are positive and 80-93 per cent are false positives. False negatives - about 10-15 per cent of women with breast cancer - meanwhile reassure women they do not have cancer when they do.
Professor Charles Wright of Vancouver Hospital and Professor C. Barber Mueller of McMaster University, Ontario, conclude that: "For most women the only 'benefit' is extra time spent with the knowledge that they have the disease.''
Two early trials of breast screening in 1982 and 1985 suggested reduction of mortality up to 30 per cent, and fuelled the campaign for mass screening. However, subsequent trials have failed to support these studies and showed no significant reduction in deaths, according to a report in today's Lancet.
A spokeswoman for the NHS Breast Screening Programme last night said that the quality of mammography in some of the studies reviewed was suspect. "All the figures quoted in this paper have been discussed before. The Canadian study has major problems and was excluded from the European Overview published by the European Society for Mastology last year."