Tens of thousands of hospital appointment follow-up letters have not been sent to GPs due to a computer system issue.
Worcestershire Acute Hospitals NHS Trust said it had a backlog of 22,000 letters, from 2011 to 2017 about patient appointments and care.
Some patients may not have received the follow-up care they should have, the chief executive said.
Patients adversely affected will be contacted by the trust.
A review is also under way.
Worcestershire Royal Hospital; Kidderminster Hospital and Treatment Centre; and Alexandra Hospital, in Redditch, are all run by the trust.
The trust, which has been rated inadequate since December 2015, 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.
An initial review into the letters error found 11,000 letters require no further medical actions.
The focus is now on the remaining letters, a statement said, which should be completed by September.
Chief executive Michelle McKay apologised that some letters within the trust's management system had not been processed properly.
"We regret that this means some patients may not have received the follow-up care they should have," she said.
"We are working closely with our primary care colleagues and partner health organisations to urgently review the individual cases of these patients and to ensure, where appropriate, patients receive the necessary follow-up care quickly."
She said an inquiry was under way to understand how this had happened.
"This is a serious issue which we are working hard to quickly address.
"However, it is important we reassure our local communities that more than half a million patients are seen in our outpatient departments each year and the vast majority of these patients will have had the appropriate letters sent to ensure they receive the right follow-up care."