A hospital failed to spot cases of lung cancer because it did not check patients' chest X-rays properly, the Care Quality Commission has found.
The health watchdog found that three patients at Queen Alexandra Hospital in Portsmouth suffered "significant harm".
It emerged that junior doctors complained they had been asked to carry out specialist radiology work without the appropriate training.
The CQC has now launched a national review into NHS radiology services.
All NHS bodies have been instructed to provide details about their backlogs, turnaround times, staffing and arrangements for routine reporting of images.
Portsmouth Hospitals NHS Trust said it had made an unreserved apology to the families of the three patients, two of whom have died.
During their visit in July, CQC inspectors also found the hospital had a backlog of 23,000 chest X-rays.
None of the 23,000 images from the preceding 12 months had been formally reviewed by a radiologist or appropriately-trained clinician.
During the visit, inspectors learned some junior doctors had been given responsibility for reviewing the chest and abdomen X-rays.
Prof Ted Baker, from the CQC, said "When a patient is referred for an X-ray or scan, it is important that the resulting images are examined and reported on by properly trained clinical staff who know what they are looking for – this is a specialist skill."
Following the inspection, the trust has had to put in place steps to make sure images are examined and reported on by properly trained clinical staff.
It was also tasked with providing a weekly report of the number of outstanding X-rays to the CQC and told it must notify those patients if their X-ray was held up.
In August, the CQC rated the hospital's medical care "inadequate".
Portsmouth Hospitals NHS Trust chief executive Mark Cubbon said: "We have issued an unreserved apology to the families of the three patients who experienced harm because of the delays to their care.
"We have carried out a thorough review of the scans and X-rays reported so far; to date nearly 50% of the backlog has been cleared and we are in touch with any patients as necessary."
The trust has set up a free phone helpline – 0800 7837118 – for anyone who is concerned they may be affected.
The hospital in Cosham has 975 beds and provides services to a local population of about 610,000 people.