The Commission for Health Improvement is to undertake its first investigation requested by somebody other than the health secretary in England or Wales, into the breast-screening service at Hammersmith Hospitals trust.
The decision to investigate the west London breast-screening service followed an external audit of over 100,000 files and a request to CHI for advice from trust chief executive John Cooper.
The external review of clinical files at Charing Cross Hospital, where the service is based, was initiated after a recall error was found in one file, and errors were uncovered in at least 11 other cases.
CHI's initial phase of data collection for the investigation is due to start within the next two weeks, and it is hoped the visit to the trust will be before August, with an intention to publish the report in the autumn.
A radiologist and a manager from London region breastscreening service have been seconded part-time into the Charing Cross unit to review administrative and managerial systems over the next few months.
The announcement of the investigation follows confirmation of the appointment of London Underground managing director Derek Smith as the next chief executive of the trust. He will take over when Mr Cooper takes early retirement in October.