NHS trusts raised concerns about breast cancer screening invitations last year but were told it was a local issue, it has been claimed.
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 London and the West Midlands raised concerns some women were not being invited for mammograms.
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 it is responsible for the blunder, which Mr Hunt said may have led up to 270 women to have 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.”
Women in England between the ages of 50 and 70 are currently 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 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.
Widower Brian Gough said his wife Trixie did not receive a letter inviting her to go for a screening in 2009.
A scan in October 2010 revealed she had stage-three breast cancer.
The 77-year-old, from Norfolk, told the Press Association he was “shell-shocked” by the news.
“There has always got to be some blame, these things don’t just happen… it is never the computer that goes wrong, it is the person that put the information in or took it out,” he said.
“Somebody somewhere along the line has made a massive error – we are talking 450,000 letters that should have gone out.”
Dame Elizabeth Anionwu, emeritus professor of nursing at the University of West London, believes she is one of those affected by the error.
The 70-year-old’s last mammogram was in 2013, but she did not receive an invitation for her next screening in 2016.
When she called the helpline she was surprised to be told she may have to wait up to six months for a catch-up screening.
“It’s not necessarily that I was panicking about getting breast cancer before then, but I had a sort of realisation of the enormity of what must be going on,” she said.
“I could imagine it could cause a lot of anxiety for people who now know they are in this group, but may have to wait until October to get the breast screening appointment.”
Dame Elizabeth, who previously worked in the NHS, said women were “reliant on the system”, adding that it had not “struck” her she had not received an appointment.
She said: “If a system has been set up, you can understand how time can pass.
“You know, we’ve got lives to live, we are not waiting every morning for our breast screening appointment.”
The official helpline for those who think they may be affected is 0800 169 2692.