Health Secretary Jeremy Hunt revealed on Wednesday that 450,000 women aged 68 to 71 had not been invited to their final routine screening.
An independent review has been launched into the computer error, which Mr Hunt said was discovered in January and dates back to 2009.
However, as early as March 2017, two breast cancer screening centres in the West Midlands, as well as one in London, raised concerns that some women were not being invited for mammograms.
It has not been disclosed, which hospitals the centres are at.
Software provider Hitachi Consulting said at the time it was a local problem and the full scale of the issue was not realised until January, Public Health England (PHE) said.
The company has denied that it is responsible for the blunder, which Mr Hunt said may have led to up to 270 women having their lives cut short.
A spokesman said: “Hitachi Consulting has no responsibility for the error that has led to this situation.
“The software responsible for inviting patients for breast cancer screening was written by others and implemented in 2009, fully six years before Hitachi Consulting began working with Public Health England.
“Hitachi Consulting was engaged in late-2015 merely to maintain and support the system as designed, and has had no responsibility for decisions made on which patients should be selected for screening.
“Our hearts go out to those people and their families who may have been affected.”
More than 8,000 calls have been made to a dedicated helpline for those affected, while charity Breast Cancer Care said it had also seen increased demand on its own helpline.
Women living in England between the ages of 50 and 70 are automatically invited for breast cancer screening every three years.
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.
However, Mr Hunt said ‘administrative incompetence’ meant that some families may have lost, or may be about to lose, a loved one to cancer.
Women and the families of those who may have been affected by the failures have demanded answers over the failures.