Thousands of hospital patients may have been discharged without medical notes being sent to them or their GP.
East and North Hertfordshire NHS Trust declared a "serious incident" after it found 14,600 people could have been affected by an issue with a new patient record system.
The system, Lorenzo, was introduced at the Lister Hospital in Stevenage in September 2017.
As reported in HuffPost, the system did not issue "discharge summaries" to GPs.
Discharge summaries are sent to GPs and provide information about a patient's treatment in hospital. The notes can include any detail about changes to medication and further appointments or tests.
'No evidence of harm'
The trust has launched a review of patient records and said it would speak "directly" to anyone who requires "follow-up action".
However, it said there was no evidence that patients had been harmed.
Stevenage MP Stephen McPartland said the incident was "shocking".
In a letter to GP practices, the trust's Medical Director Michael Chilvers said the trust "takes full responsibility for what has happened".
The trust has advised anyone worried about their discharge summary to contact the patient advice and liaison service.