At least 1,700 patients may have been harmed by a major blunder which saw more than 700,000 pieces of confidential medical correspondence mislaid, according to a new report.
The study, from the National Audit Office (NAO), found that the Department of Health, which co-owns the company responsible for the error, did not inform Parliament or the public for several months about the issue.
One reason was because Department officials were worried that publicity may harm insurance arrangements, making it potentially liable for any compensation claims from patients.
The blunder, which covers five years up to early 2016, relates to many types of medical documents, including treatment plans, blood and urine tests and cancer diagnoses.
Sensitive documents relating to child protection - some needing immediate action - were also included.
The documents, sent between GPs and hospitals, did not reach their recipients because they were piled up in a warehouse by NHS Shared Business Services (SBS), which is co-owned by the Department of Health and private firm Sopra Steria.
The NAO report found that, as of May 31 this year, a review of the backlog has found 1,788 cases of potential harm to patients.
Overall, NHS England and NHS SBS have identified just under 709,000 items of unprocessed correspondence.
The study said that while no cases of actual harm have been identified yet, a third of GPs have not yet responded on whether unprocessed items sent to them indicate potential harm for patients.
Liberal Democrat leader Tim Farron said: "This was a colossal blunder, which has put 1,788 people in harm's way - and this figure could be much, much higher."
He accused the Government of trying to "simply wave this away".
He added: "This disaster left patient data, which includes blood test results and cancer screening, languishing in a warehouse. People in the Department must be held accountable for this shoddy affair."
Health Secretary Jeremy Hunt became aware of the blunder in March 2016 when health bosses discovered thousands of pieces of mail had not been sent on by NHS SBS, which was operating a redirection service in the East Midlands, the South West and north-east London.
The NAO report said that over a four-year period, local teams and reviews within NHS SBS observed the backlog "continuing to grow".
It said: "In January 2014, a review by NHS SBS found a backlog of around 205,000 items across the three regions and highlighted the clinical risk to patients of notes not being with GPs."
The report said: "Senior managers within the NHS SBS primary care services business unit knew about the clinical risk to patients in January 2014 but it did not develop a plan to deal with the backlog."
In June and July 2015, an NHS SBS administrator raised concerns with NHS SBS senior managers about the backlog in the East Midlands.
The same administrator raised further concerns in August that the backlog was being destroyed, with evidence that sacks of mail were being got rid of.
An NHS SBS senior manager is also said to have removed a label from a room saying "clinical notes" because "you don't want to advertise what's in that room".
It was only in January 2016 that the company's information governance manager rated the issue as a serious incident requiring investigation.
"An internal investigation found that staff had considered the misdirected clinical correspondence a lower priority than other work as there were no performance indicators attached to it," the NAO report said.
NHS England and the Department of Health were informed in March 2016, but "the Department of Health decided in April 2016 not to alert Parliament or the public to the incident", it went on.
"It considered that it did not have an accurate picture of the scale of the incident or of the potential harm to patients.
"It believed there was a risk of questions that the Department could not yet answer, potentially leading to unnecessary worry among patients and the public."
It was also worried about invalidating insurance, the study said.
The report also pointed to the conflict of interest created by the fact Mr Hunt oversees the NHS and also sits on the board of NHS SBS.
The Department finally informed Parliament of the incident in late July 2016.
To date, of 2,508 items of mail classified as high priority, GPs have assessed 2,159 of those as having "no patient harm" and are still examining the remaining items, with 229 classified as "potential harm".
GPs have yet to respond on 175,000 items of correspondence assessed by NHS England to be lower priority, despite having received payment to review these additional items in February 2017, the NAO said.
NHS England estimates the cost of the incident will be at least £6.6 million for administration alone, the report added.
A Department of Health spokeswoman said: "As the NAO report highlights, patient safety has been our first priority and no cases of harm have been identified to date.
"Alongside NHS England, we have been very mindful of appropriate transparency while working to make sure this does not happen again.
"Last year, the Health Secretary updated Parliament and the Public Accounts Committee was informed."
An NHS England spokesman said: "NHS England was deeply concerned to be belatedly informed by SBS in March 2016 about its backlog of unprocessed correspondence.
"We immediately set up a team, including clinical experts, to manage the incident, and all relevant correspondence has now been sent back to GPs for review.
"None of the patients whose cases have been reviewed to date have been harmed by the delay in correspondence."
Shadow Health Secretary Jonathan Ashworth said: "This is a total scandal. For a company partly owned by the Department of Health and a private company to fail to deliver half a million NHS letters, many of which contained information critical to patient care, is astonishing.
"There remain over 1,700 incidents of possible patient harm unaccounted for. This is a staggering catalogue of mistakes on this Government's watch."
He added: "The Secretary of State needs to explain to the public how he got himself into this conflict of interest, why the oversight of the company went wrong, and why he failed to pick up this string of mistakes for so many years. He should apologise to patients and to taxpayers."