Thousands of NHS patients may have come to harm because of an administrative mix-up, says NHS England.
Some 500,000 documents containing medical information, including cancer test results, were mistakenly put in storage rather than being sent to the GP or filed in the patients' records.
An investigation is under way, focusing on the estimated 2,500 patients who may have been adversely affected and need further medical checks.
So far, no harm has been reported.
The error occurred when a mail redirection company hired by the NHS failed to pass on documents that had either been incorrectly addressed or needed re-routing because the patient had moved to a new GP surgery.
The company, NHS Shared Business Services, has expressed regret for the failings, which occurred between 2011 and 2016 in the East Midlands, the South West and north-east London.
An NHS England spokesperson said: "A team including clinical experts has reviewed that old correspondence and it has now all been delivered wherever possible to the correct practice."
Shadow health secretary Jonathan Ashworth described the "astonishing" failure as an "absolute scandal".
"The news is heartbreaking for the families involved and it will be scarcely believable for these hospitals and GPs who are doing their best to deliver services despite the neglect of the government. We urgently need to know how this was allowed to happen, how many patients were involved and how many have been harmed, and whether patients remain at risk."