NHS England has launched an enquiry into how half a million pieces of medical correspondence were mislaid over five years and whether anyone was harmed or died from the error.
Between 2011 and 2016, some 500,000 pieces of patient data, which included test results and diagnoses sent between GPs and hospitals, did not reach their intended destinations because they were stored in a warehouse instead of being delivered, the Guardian reports.
A 50-strong team of administrators, based in Leeds, is looking at the impact of the correspondence mislaid by NHS Shared Business Services (NHS BHS) which was responsible for internal post within the health service.
GPs are cross-checking correspondence with patients' medical records. The British Medical Association says some may have taken extra drugs or had their diagnoses delayed.
"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," Richard Vautrey, chair of the BMA's GPs committee told the Guardian.
Shadow health secretary Jonathan Ashworth was critical of Jeremy Hunt, the health secretary, for a "complacent" statement to MPs when he first informed them last July about the loss of "some correspondence".
Ashworth said: "Patient safety will have been put seriously at risk as a result of this staggering incompetence."
An NHS England spokeswoman said that a team including clinical experts has reviewed old correspondence, which has now all been delivered wherever possible to the correct practice.
A Department of Health spokesperson said: "The department and NHS England have been completely transparent while work has been ongoing to resolve this issue, with patient safety as ever our first priority. In July, the health secretary informed parliament and in September, senior civil servants updated the public accounts committee."
'Stop the NHS bickering'
Separately, MPs in the Public Accounts Committee (PAC) have urged the Prime Minister, Theresa May, and the head of NHS England, Simon Stevens, to stop their public disagreements over funding.
With Number 10 aides accusing Stevens of lacking "enthusiasm" in his role, Stevens then criticised May at the committee hearing for saying the NHS had been given more money than it had asked for.
The PAC said such "contradictory statements" were an "insult to taxpayers", adding: "The fact that key players running our NHS are bickering in public does little to inspire confidence that patients are at the heart of everyone's priorities."
The report found that the deficit in the finances of the NHS had risen to £2.5bn, up from £859m the previous year, the Telegraph reports.