More than 1,800 high priority items, such as patient screening or test results, have been identified by NHS England among a backlog of 374,000 pieces of clinical correspondence which were not redirected as they should have been.
The National Audit Office (NAO), which is investigating the blunder, said it came about after a "small proportion" of GPs had not been complying with the latest guidance to return to sender if they were wrongly sent correspondence - including clinical papers, child protection notes, treatment plans and changes to patients' medication regimes - such as when a patient has moved to another practice.
Instead they have been sending it on to outsourcing firm Capita, the provider of primary care support services for NHS England, which is not contracted to forward it on.
The NAO said the issue was not formally reported to NHS England by Capita until October 2016, nearly 18 months after the new arrangements for handling misdirected correspondence were introduced in May 2015.
At the time Capita estimated that there were 580,000 items of "clinical notes", which the company admitted with hindsight it could have reported sooner.
Initial checks on the backlog identified an estimated 170,000 items of clinical correspondence, which after reviewing a small sample, NHS England considered to be of low risk, and advised internally that Capita should simply send it on the relevant GPs.
However, NHS England did not ask or contract Capita to return the correspondence.
By July 2017 Capita and NHS England had identified and logged 277,000 items of clinical correspondence, part of an agreed process to return correspondence to the correct GP.
The NAO said NHS England's National Incident Team had now identified and clinically reviewed 374,000 items of unprocessed clinical correspondence, the vast majority of which required no GP action to be taken.
NHS England identified 1,811 high priority items - such as documents deemed to be related to screening or urgent test results - and 25,361 low priority items.
By November 20 2017, NHS England sent 18,829 items of misdirected clinical correspondence to relevant GPs so they could assess whether there had been any actual harm to patients, while another 8,343 items for patients who were deceased or did not have a GP, were also assessed.
It will cost NHS England an estimated £2.4 million to review the correspondence for evidence of harm - although no actual harm has been identified yet.
It is expected to be completed by the end of March - but NHS England has not yet been able to stop GPs from sending clinical correspondence to Capita in error, and NHS England has continued to receive between 5,000 and 10,000 items of clinical correspondence from Capita a month.
The NAO said it expected NHS England to carry out an information campaign to ensure that GPs understood the guidance for handling correspondence for patients who are not registered at their practice.
Dr Richard Vautrey, British Medical Association (BMA) GP committee chairman, said: "It's no surprise that with the now longstanding confusion, chaos and failures of these support services for general practice, that a small number of practices may have, in good faith, sent on misdirected correspondence.
"What is clear from this investigation is the inability of both NHS England and Capita to get to grips with a problem they have now known about for years.
"Capita itself admitted it failed to officially disclose the scale of the backlog to NHS England in good time, and both are yet to decide whose responsibility it is to return records to the right practice.
"Further, NHS England has still not launched an effective information campaign aimed at GPs, despite continuing to receive misdirected correspondence almost two years after it became aware of the problem.
"This is a further indictment of Capita's shambolic running of GP backroom services, and the real effect its failings are having on the safe care of patients."