The error emerged when a practice received a year-old letter.
HEALTH chiefs are investigating whether patients died or came to harm after a backlog of 22,000 letters went undelivered for up to six years.
Worcestershire Acute Hospitals NHS Trust said the follow-up notes should have been sent to GPs and other departments from 2011 to 2017.
But some staff were unaware they had to click two onscreen buttons to send the messages, meaning they were never dispatched.
The letters were written following hospital outpatient appointments, outlining the treatment or tests the patient had received or the follow-up care they needed.
A number of patients are since thought to have died but it is not known if this was as a result of the blunder.
Affected patients will be contacted by the trust and a review is under way.
Worcestershire Royal Hospital; Kidderminster Hospital and Treatment Centre; and Alexandra Hospital, in Redditch, are all run by the trust.
The trust has been rated inadequate since December 2015 and was criticised earlier this week by the Care Quality Commission.
Its latest report found patients were being cared for in emergency department corridors as standard practice and no “tangible improvements” had been made since an earlier inspection in November.
The backlog came to light when the St Stephen’s practice, in Redditch, received a letter which was a year old. It was one of 69 letters – none of which were critical.
An initial review found 11,000 letters require no medical action.
The Trust is working with general practices to review the remaining letters, which should all be assessed by the end of September.