Up to 270 women in England may have died because they did not receive invitations to a final routine breast cancer screening, the Health Secretary Jeremy Hunt says.
Speaking in the Commons, he said 450,000 women aged 68-71 had failed to get invitations since 2009.
Mr Hunt has announced an independent review and apologised to the women and their families.
He said oversight of the screening programme had “not been good enough”.
GPs leaders said they were “shocked” to learn of the error and said the implications for GPs would potentially be “significant”.
Of the 450,000 women affected, 309,000 are still alive and in their 70s.
Mr Hunt said: “For them and others it is incredibly upsetting to know that you did not receive an invitation for screening at the correct time and totally devastating to hear you may have lost or be about to lose a loved one because of administrative incompetence.”
He said a computer algorithm failure was to blame, which meant women who had just turned 70 were not sent an invitation for a final scan as they should have been.
Mr Hunt said computer modelling suggested that between 135 and 270 women may have had their lives shortened.
He said variation in the way that local services sent out invitations may also have been a factor.
“It is a major priority to do our very best to make sure that the additional scans do not cause any delays in the regular breast screening programme for those under 71,” he said.
“NHS England have taken major steps to expand the capacity of screening services.”
Mr Hunt said: “Irrespective of when the incident started, the fact is for many years oversight of our screening programme has not been good enough.
“Many families will be deeply disturbed by these revelations, not least because there will be some people who receive a letter having had a recent diagnosis of breast cancer.”
Women in Scotland are not affected by the error because it has a different IT system. Wales and Northern Ireland have similar systems to England but Mr Hunt said there was no reason to believe they were affected.
All women affected will now be contacted by letter by the end of May and those under 75 will be offered a routine catch-up mammogram.
A helpline will also be set up for women aged over 72 to talk them through the pros and cons of having breast screening – scans in older women sometimes pick up cancers which do not require treatment.
Any women who wants a mammogram will get one within six months, Mr Hunt said.
The Royal College of GPs urged women affected not to panic. It said they should contact a national helpline which has been set up – 0800 169 2692 – or go to the NHS Choices website for more information.
Breast cancer screening is currently offered to women aged 50 to 70 in England.
All women in this age group registered with a GP are invited for screening every three years.
This is because the likelihood of getting breast cancer increases with age.
After 70, women can still have screening if they choose to by contacting their GP.
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