About 800 patients at a GP practice have been told errors have been found in their records, leaving the "safety" of their care potentially at risk.
People at Station Surgery in Leyland, Lancashire, will be asked to provide "further information" to help amend "inaccuracies" in their medical notes.
A letter informed them the matter may affect "the quality and safety of your care".
No specific details of the nature of the inaccuracies have been released.
However, the Local Democracy Reporting Service understands an ongoing investigation is focusing on patients with chronic diseases, who make up about half of the patients on the surgery's books.
The records of these patients are being checked first.
Any further patients who are directly affected, beyond the 800 identified, will also receive a letter with details of how an appointment can be arranged.
Previous patients contacted
Other practices caring for patients previously registered with Station Surgery have also been contacted.
Helen Curtis, director of quality and performance at Chorley and South Ribble clinical commissioning group, said: "We are satisfied that arrangements are now in place to make sure such issues do not occur again."
The iquriy is being led by the Preston-based Park View Surgery, which temporarily took over the running of the practice in May, after the sudden death of its sole GP, Dr. George Ahad, in April.
The 75-year-old had run the surgery for 31 years.
It is understood the issues came to light shortly after Park View took control.
The clinical commissioning group has decided to keep the surgery open under the management of Park View, until a permanent provider is found next year.