The new results are based on a study of more than 160,000 women who were enrolled in the UK Breast Screening Age Trial between 1990 and 1997, when they were aged between 39 and 41.
They have been followed up for 23 years to see if early screening made a difference, and researchers found mortality dropped by a quarter in the first 10 years.
The total years of life saved from breast cancer in the intervention group was estimated as 620, corresponding to 11.5 years saved per 1,000 women invited to earlier screening.
The researchers estimate that 1,150 women would have to be screened in their 40s to prevent one breast cancer death.
The results also suggest that any overdiagnosed cancers would likely also be diagnosed at NHS screening from 50 years of age.
Commenting on the research, Shirley Hodgson, professor of cancer genetics at St George’s University of London, said: “The clear implication from this study is that screening from 40 years does appear to save lives from breast cancer, particularly early stage (one and two) breast cancer.
“The fact that early screening involves annual screens, which at the time of the study were less sensitive than those done nowadays, indicates that the sensitivity of screening may be greater now, but possibly this could result in more false positive diagnoses.
“There is clear evidence from this study that mammography screening from 40 years of age has the potential to save lives from breast cancer.”
However, Kevin McConway, emeritus professor of applied statistics at the Open University, said there was still a danger from overdiagnosis.
Currently, for every one woman whose life is saved by the current screening programme, three women are diagnosed with a cancer that would never threaten their lives.
“An earlier overdiagnosis does mean that a woman might have to live with the consequences of unnecessary treatment for longer,” said Prof McConway.
“It’s also the case, on the current screening programme, that most women whose mammogram shows something suspicious, so that they need further investigation, actually turn out not to have a cancer – currently that’s true for three out of every four women whose mammogram looks abnormal.”
The authors say they have not yet considered the cost-effectiveness of screening earlier.
The results were published in The Lancet Oncology.