Health Secretary Jeremy Hunt has said there is "no evidence" that patients have been put at risk after the NHS mislaid more than 700,000 pieces of confidential medical correspondence.
Up to five years' worth of documents were not delivered after being mistakenly left in an NHS England warehouse run by private company NHS Shared Business Services (SBS).
The contents of the messages, which were sent between GPs and hospitals between 2011-2016, ranged from blood and urine test results to cancer diagnoses and treatment details.
Mr Hunt confirmed more than 708,000 documents had failed to be delivered but said so far no patient harm has been identified as a result of the huge misplacement.
Responding to an urgent question in the House of Commons, he said a serious incident team had examined almost all the files since the mistake emerged in March 2016 at a cost to the taxpayer of £2.2 million.
Mr Hunt said 2,000 of the 2,500 files that needed greater investigation had now been assessed as posing no harm to patients with the remaining 500 files still being assessed.
"As things stand ... there is no evidence so far that any patient safety has been put at risk," he told MPs.
The health secretary denied an accusation from shadow health secretary Jonathan Ashworth that the mislaying of files amounted to a "catastrophic breach", saying no patient data was lost.
Mr Ashworth also accused Mr Hunt of failing to disclose the scale of the problem when he confirmed an undelivered backlog existed in a 138-word statement in July 2016, four months after the error was first discovered.
The health secretary had then said "some correspondence in the mail redirection service has not reached the intended recipients".
A report in The Guardian that the NHS had been quietly reviewing how many patients had been affected by the error for almost a year raised allegations of a "cover up".
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.
SBS, which is co-owned by the Department of Health and French company Sopra Steria, was operating the redirection service in the East Midlands, the South West, and north-east London.
Mr Ashworth cited accusations of a "cover-up" as he pressed the Health Secretary on why the full extent of the problem was not revealed last year.
He added: "Two months into 2017 and the Health Secretary lurches from one crisis to another. Hospitals overcrowded, waiting lists out of control.
"He can't deliver the investment our NHS needs, he can't deliver a social care solution, he can't deliver patient safety and now he can't even deliver the post."
Mr Hunt said he had made his original statement to the house despite being advised by NHS officials not to publicise the matter.
Richard Vautrey, deputy chairman of the British Medical Association's GP committee, warned patients could have been harmed by the failure.
"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 said.
An NHS England spokesperson said a team "including clinical experts" had reviewed the more than 700,000 files adding, "It has now all been delivered wherever possible to the correct practice. SBS have expressed regret for this situation."