At least 10 people have died after the NHS failed to invite more than 5,000 people for routine cancer screenings, health officials have revealed.
The error - which dates all the way back to 2008 - caused thousands to miss vital cancer checks as a result of incomplete GP registrations.
A further 10 people who are still alive have been diagnosed with cancers that screening might have detected earlier.
The NHS has now written to 5,261 affected individuals after the mistake came to light once people began to report that they had not received screening invitations.
Department for Health and Social Care, London
PA
Public health minister Ashley Dalton explained the error happened when patients' GP registration process was "not completed correctly, meaning their details were not passed to the NHS screening system".
The issue affected bowel, breast and cervical cancer screening programmes, as well as abdominal aortic aneurysm screenings.
Records indicate that up to 10 patients have been diagnosed with a relevant cancer and were not invited for certain screening.
"The impact on these patients is not yet known and a clinical harm assessment process will be undertaken, based on expert clinical advice," Dalton said in her statement.
She added: "It is with deep sadness that I must report that records also indicate that around 10 people who were not invited for screening may have died from a relevant cancer."
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NHS England's national director for vaccinations and screening Steve Russell confirmed the health service is contacting all those affected.
Additionally, the NHS has established a helpline for those concerned they may have missed screening invitations.
The Department of Health and Social Care said it would work closely with NHS England to ensure this never happens again.
Russell extended "sincere apologies to those affected for this error and any additional worry this may have caused", adding that new guidance had been issued to GP practices to ensure patient registrations are fully completed.
The error affected bowel, breast and cervical cancer screening programmes from 2008 (Stock)
PAThe NHS has confirmed the issue does not affect the diabetic eye screening programme or any antenatal or newborn screening programmes.
A Department of Health and Social Care spokesman said: "We apologise to everyone who has been affected by this issue. This should have never happened, and we will work closely with NHS England to ensure it never does again."
Executive director of policy at Cancer Research UK Dr Ian Walker described the blunder as "unacceptable".
"It's unacceptable that some people haven't received an invitation, and we welcome NHS England's announcement that there will be a full review to understand how this happened," he said.
The NHS has confirmed that processes have been put in place to ensure new GP registrations will be closely monitored and updated on systems as necessary.
