Screening women in their 40s for breast cancer cuts death rates from the disease by 17 per cent, but may still not be worth doing, researchers say.

Regular screening for women under 50, like that offered by the NHS to women over 50, does saves lives, but the reduction in deaths was not statistically significant and it caused harm, a study has shown.

If the NHS were to introduce screening for the under-50s, many women would have unnecessary investigations, and there could be a risk of cancer caused by the radiation from the screening, scientists from the Institute for Cancer Research say. Eight of 10 breast cancers occur in women over 50 and the NHS breast-screening service is said to save 1,000 lives a year.

The Institute launched a 10-year trial of 160,000 women aged from 40 to 50. The results, in the Lancet, show the one third offered annual breast-screening had fewer deaths from breast cancer than the two thirds who had usual medical care.