A review into radiology services across the NHS in England has been launched after it emerged more than 20,000 x-rays at one hospital were not properly examined.
Inspectors at the Care Quality Commission have acted after it emerged junior doctors were left to interpret chest x-rays, including those for suspected cancer, at Queen Alexandra Hospital in Portsmouth.
As part of the national review, NHS bodies across the country have been ordered to provide the commission with information on their backlogs, turnaround times, staffing and arrangements for the routine reporting of images.
Inspectors discovered three "serious incidents" at Queen Alexandra after a member of the public raised concerns, including two where lung cancer had possibly spread because inexperienced doctors were left to look at scans.
In both cases, the patients attended as emergency cases and were sent for a chest x-ray, but neither of them got a full radiology report, according to the CQC.
In the first case, a junior doctor looked at the x-ray and said "no abnormality was detected". But a year later, when the patient was referred by their GP for another examination, a radiologist found lung cancer.
The expert said the "abnormality" was evident in the first x-ray and should have been spotted then.
The second case was very similar, the CQC said, while the third case is still under investigation.
Inspectors said: "The delay in diagnosis caused significant harm to both patients."
The CQC discovered that, starting in 2007, chest x-ray reports started being delegated to the medics who had requested them amid an "escalating problem with the lack of capacity within radiology".
By the time the CQC got involved, the only chest x-rays that were reviewed by radiologists were ones asked for by GPs, nurses and physiotherapists, as well as x-rays on children.
Between 1 April 2016 and 31 March 2017, 26,345 chest x-rays and 2,167 abdomen x-rays were not looked at by a radiologist or appropriately trained clinician, the CQC found.
The trust has been told to take immediate action to make sure x-rays are reviewed by an appropriately trained clinician.
CQC's chief inspector of hospitals Professor Ted Baker 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.
"During our inspection of Portsmouth Hospitals NHS Trust, however, some junior doctors told us that they had been given responsibility for reviewing chest and abdomen x-rays without appropriate training although they felt that they were not competent or confident to do so.
"We then learned of some cases where signs of lung cancer were missed, with serious consequences for the patients involved. This is clearly unacceptable."
Mark Cubbon, chief executive of Portsmouth Hospital NHS Trust, said the hospital had apologised to the families of the three patients concerned and takes the CQC's findings "extremely seriously".
He said: "It is of deep regret to all of us that we did not deliver the high standards of care everyone should expect from our hospital."