"Cancer charities and public health authorities have been misleading women for the past two decades by giving too rosy a picture of the benefits," said Karsten Jorgensen, a researcher at the Nordic Cochrane Centre in Copenhagen who has previously published papers on overdiagnosis.
"It's important they have at least acknowledged screening causes substantial harms," he said, adding that countries should now re-evaluate their own breast cancer programs.
In the U.S., a government-appointed task force of experts recommends women at average risk of cancer get mammograms every two years starting at age 50. But the American Cancer Society and other groups advise women to get annual mammograms starting at age 40.
In recent years, the British breast screening program has been slammed for focusing on the benefits of mammograms and downplaying the risks.
Maggie Wilcox, a breast cancer survivor and member of the expert panel, said the current information on mammograms given to British women was inadequate.
"I went into (screening) blindly without knowing about the possibility of overdiagnosis," said Wilcox, 70, who had a mastectomy several years ago. "I just thought, 'it's good for you, so you do it.'"
Knowing what she knows now about the problem of overtreatment, Wilcox says she still would have chosen to get screened. "But I would have wanted to know enough to make an informed choice for myself."