Research has suggested that breast cancer death rates may not be affected by screening programmes.
Danish experts have shown that there are few differences in women who are screened and those who are not screened when it comes to death rates from breast cancer, casting doubt over the benefits of mammography.
In research conducted last July the team also found that one in three breast cancers detected by screening may actually be harmless.
That study, based on data from the UK, Canada, Australia, Sweden and Norway, suggested some women undergo unnecessary treatment for cancers that are unlikely to kill them or spread.
Some cancers grow so slowly that the patient dies of other causes first, or the cancer remains dormant or regresses.
The researchers, from the Nordic Cochrane Centre in Denmark, said cancer screening programmes could lead to "overdiagnosis".
In their latest 10-year study, published online in the British Medical Journal (BMJ), experts from the Centre found death rates from breast cancer fell 1% per year in screened areas among Danish women aged 55 to 74 and by 2% in non-screened areas.
In women too young to benefit from screening (aged 35 to 54), breast cancer death rates fell by 5% per year in the screened areas and by 6% per year in the non-screened areas.
For older age groups (75 to 84), there was little change over time both in screened and non-screened areas.
"I am so glad to have had a mammogram last March aged 50yrs. I was diagnosed with a small grade2/3 cancer in my rt breast. There was no lymph node spread and I didn't have to have chemotherapy.I would have been unhappy not to have this treated and even more unhappy not to have been offered a mammogram.How can the research discard this kind of evidence?" - Helen Campbell, Brighton