The NHS has mislaid more than half a million pieces of patients' confidential medical correspondence, including treatment plans and cancer test results.
The documents, sent between GPs and hospitals over five years, did not reach their recipients because they were mistakenly stored in a warehouse by private company NHS Shared Business Services (SBS).
There have been allegations of a "cover up" after The Guardian reported that NHS England has been quietly reviewing how many patients had been affected by the error, which was discovered in March 2016.
Richard Vautrey, deputy chairman of the British Medical Association's GP committee, told the newspaper that patients could have been harmed by the failure, which began in 2011.
"This is a very serious incident, it should never have happened and it's an example of what happens when the NHS tries to cut costs by inviting private companies to do work which they don't do properly," he added.
The contents of the more than 500,000 misplaced messages ranged from blood and urine test results to cancer diagnoses.
The NHS is now spending millions of pounds assessing the scale of the situation and will probe whether the delays contributed to any patients' deaths, The Guardian said.
SBS, which is co-owned by the Department of Health and French company Sopra Steria, was operating a redirection service in the East Midlands, the South West and north-east London.
The issue relates to patients' documents not being forwarded after they were sent to practices where the patient was not known or had moved away.
Liberal Democrat leader Tim Farron said: "This looks, to me, like a cover up. (Health Secretary) Jeremy Hunt has serious questions to answer.
"This is a staggering loss of personal, private data. This colossal loss of vital material may well have crucially impacted on a patient's treatment.
"I worry that while all this correspondence - including test results - has gathered dust, patients been put at risk.
"People could have died as a result of this, and so we need to know who knew what and when."
Shadow health secretary Jonathan Ashworth described the "astonishing" failure as an "absolute scandal".
"Patient safety will have been put seriously at risk as a result of this staggering incompetence," he added.
"Jeremy Hunt now needs to tell the whole truth of why patient safety has been put at risk in this way."
An NHS England spokesman said: "Some correspondence forwarded to SBS between 2011-2016 was not redirected or forwarded by them to GP surgeries or linked to the medical record when the sender sent correspondence to the wrong GP or the patient changed practice.
"A team including clinical experts has reviewed that old correspondence and it has now all been delivered wherever possible to the correct practice.
"SBS have expressed regret for this situation."