Most women should start regular breast cancer screening at age 50, not 40, according to new guidelines recently released by an influential group that provides guidance to doctors, insurance companies and policy makers.

The new recommendations, which do not apply to a small group of women with unusual risk factors for breast cancer, reverse longstanding guidelines and are aimed at reducing harm from overtreatment, the group says.

It also says women age 50 to 74 should have mammograms less frequently — every two years, rather than every year. And it said doctors should stop teaching women to examine their breasts on a regular basis.

Its new guidelines, which are different from those of some professional and advocacy organizations, are published online in The Annals of Internal Medicine and are likely to touch off yet another round of controversy over the benefits of screening for breast cancer.