Across the UK, 1,700 patients have so far been identified in the 700,000 notes found - with the possibly that many more could come to light.
The company responsible for the error - NHS Shared Business Services (SBS) - was employed in the East Midlands, South West and north-east London to redirect mail for the health service.
However, between 2011 and 2016 a backlog of 709,000 pieces of correspondence - including cancer test results and child protection notes - piled up in their warehouse.
A spokeswoman for NHS SBS admitted to the BBC that there had been ‘failings’.
She added: “We regret this situation and have co-operated fully with the National Audit Office in its investigation.”
The issue first came to light in February after it was reported by the Guardian newspaper.
Investigations overseen by NHS England are expected to be completed by the end of the year.