Health secretary Jeremy Hunt has announced that around 450,000 women in England missed out on breast cancer screening due to a technical error.
Between 135 and 270 women may have “had their lives shortened ” because they were not sent invitations to a routine check-up, he said.
Apologising, he said a public inquiry into the NHS breast cancer screening programme would be launched.
Women who missed appointments will be contacted and offered a screening, he added.
Here is what to do if you think you were among hundreds of thousands affected.
Who is affected by the problem?
The NHS breast screening programme invites more than 2.5 million women every year to come in for a check-up.
Women between the ages of 50 to 70 are tested every three years up to their 71st birthday. Around 2 million women take up the offer annually.
In January 2018, Public Health England analysis found hundreds of thousands of women had not been sent invitations to their final routine screening appointment between their 68th and 71th birthdays.
Around 450,000 women missed out on a check-up between 2009 and this year.
How do I know if I missed a screening appointment?
Of the affected group, 309,000 women are believed to still be alive.
The NHS is writing to every women who is registered with a GP and did not receive their final routine appointment. It will begin sending out letters immediately, with 65,000 going out this week.
It is aiming to have contacted each woman affected by the end of May.
Only women in England missed appointments because of the issue.
Mr Hunt said Scotland’s health services use a different IT system from NHS England and while Wales and Northern Ireland use a similar system, neither is believed to have been affected by the problem.
Affected women who have moved out of England to another part of the UK will still be contacted.
“We hope to reassure anyone who does not receive a letter this month that they are not likely to be affected,” the health secretary said.
The families of women who missed a check-up and have since died of breast cancer will also be notified.
Mr Hunt said: “As well as apologising to families affected, we wish to offer any further advice they might find helpful, including a process to establish whether a missed scan was a likely cause of death and compensation is therefore payable. We recognise this will be incredibly distressing for some families.”
What should I do if I missed a check-up?
All women were missed out on their final routine appointment will be given the opportunity of catch-up screening.
Women under 72 will be automatically invited to a check-up and sent a letter informing them of the date and time.
Those above that age will be given the number for a dedicated helpline to discuss whether screening could benefit them.
For older women, there is a greater risk that screening will pick up benign cancers that could lead to them undergoing unnecessary and harmful tests and treatment.
Mr Hunt said NHS England had "taken major steps to expand the capacity of screening services” to ensure all the affected women can be screened by the end of October.
However, he stressed “in the vast majority of cases it will be much sooner than that”.
What should I do if I’m concerned about breast cancer?
Public Health England has set up a helpline on 0800 169 2692 for women to call for advice.
Charities including Breast Cancer Care and Macmillan Cancer Support also have helplines and offer information about screening on their websites.
Although women aged over 70 are not automatically invited to screening, they are still eligible for check-ups if they want them.
Emma Greenwood, Cancer Research UK’s director of policy and public affairs, said: “If you suspect you have been directly affected by this or if you are over 50 and haven’t had a mammogram in the last three years and would like one, the NHS Choices website provides further information and the option to contact your local unit to book an appointment.”
