Scientists say the "jury is still out" following a 10-year investigation into whether women aged 40 to 50 should be screened for breast cancer.
Currently women aged 50 to 70 are invited for screening
The Department of Health-backed study, involving 160,000 women, found yearly screening made no impact on breast cancer deaths in this age group.
Currently, when women reach 50 they are invited for screening every three years by the NHS Breast Screening Programme.
The Institute of Cancer Research report is published in the Lancet.
In the study, funded by Cancer Research UK, a third of the women received annual screening invitations at the age of 40 and two-thirds only the usual three-yearly screening invite at the age of 50.
The women were then followed up for an average of 11 years.
From their findings, the researchers estimate screening from age 40 could save four lives for every 10,000 women screened.
But the benefit of screening women in their 40s needs to be balanced against possible negative considerations, they say.
For example screening at younger ages would increase a woman's radiation exposure, which can contribute to breast cancer risk.
And recalling women who do not have cancer for further tests, adds to their anxiety and results in higher financial costs for the screening programme.
Weighing up the risks
The report authors, Dr Sue Moss and colleagues, say the potential benefits of screening the under 50s could actually be greater than that they observed, particularly with longer follow up.
Professor John Toy, medical director of Cancer Research UK, agreed, saying: "More years of follow-up might reveal a benefit."
Dr Sarah Rawlings of Breakthrough Breast Cancer, said: "We'd like to see further research in this age group as we know many women want information on whether screening between these ages would be helpful."
"Eight out of 10 breast cancers occur in women over 50 and we'd urge these women to attend breast screening.
"Women of all ages should be breast aware and report any unusual changes to their doctor. The earlier breast cancer is caught, the easier it is to treat."
Julietta Patnick, director of the NHS Cancer Screening Programmes, said: "It is important that the benefits and limitations of screening this age group are properly evaluated.
"The advisory committee on breast cancer screening will consider the findings in detail."