An independent review into the breast screening scandal has been launched after it was revealed that a computer failure meant that women were not invited for their final routine screening.
Health Secretary Jeremy Hunt launched the review after revealing that the error, dating back to 2009, meant women aged 68 to 71 in England were not invited to their screening.
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He admitted 450,000 women could be affected, and that between 135 and 270 women could have had their lives shortened as a result.
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Baroness Delyth Morgan, chief executive at Breast Cancer Now, said it was a 'colossal systemic failure'.
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Yesterday, Mr Hunt said 'administrative incompetence' meant some families may have lost, or may be about to lose, a loved one to cancer.
Women in England between the ages of 50 and 70 are currently automatically invited for breast cancer screening every three years.
The issue was first brought to the attention of the Department of Health and Social Care in January, but was initially thought to pose a 'limited' risk to patients.
It was escalated to ministers in March following an urgent clinical review, with the Government told the error should not be made public to ensure existing screening services were not overwhelmed.
Baroness Morgan said Breast Cancer Now welcomed the inquiry to ensure similar mistakes cannot be made.
She said: "It is beyond belief that this major mistake has been sustained for almost a decade and we need to know why this has been allowed to happen.
"We are deeply saddened and extremely concerned to hear that so many women have been let down by such a colossal systemic failure.
"That hundreds of thousands of women have not received the screening invitations they've been relying upon, at a time when they may be most at risk of breast cancer, is totally unacceptable."
Of those who missed invitations, 309,000 are estimated to still be alive and all those living in the UK who are registered with a GP will be contacted before the end of May.
All women who were not sent an invitation for their final screening will be given the opportunity to have a new appointment.
Those under the age of 72 will receive an appointment letter informing them of the time and date, while those over 72 will also be offered a screening and have access to a helpline to decide if it will be beneficial.
The helpline for those who think they may be affected is 0800 169 2692.