A national review has been launched into radiology services in the NHS after it emerged patients came to "significant harm".
More than 20,000 x-rays had not been reviewed by a radiologist or an appropriately trained clinician at one hospital.
And junior doctors were left to interpret chest x-rays, including those for suspected cancer at Queen Alexandra Hospital in Portsmouth.
Inspectors found three "serious incidents" involving patients at the hospital after a member of public raised concerns.
The incidents included two where lung cancer had possibly spread due to inexperienced doctors being left to interpret scans.
Two patients attended as emergency cases and were sent for a chest x-ray but neither received a formal radiological report, the CQC said.
In the first case, a junior doctor interpreted the x-ray and reported that "no abnormality was detected".
A year later, after a GP referral for an x-ray, a radiologist detected lung cancer.
When re-reviewing the initial x-ray, the expert also felt the "abnormality" was evident in the previous chest x-ray, taken a year earlier.
The second case was very similar, the CQC said.
The Care Quality Commission (CQC) is now reviewing radiology reporting across the NHS in England.
All NHS bodies have been ordered to provide details on their backlogs, turnaround times, staffing, and arrangements for routine reporting of images.
It has ordered the trust to take immediate action to ensure that x-rays are reviewed by appropriately trained clinicians.
"The notes showed no formal review by the referrer, so it is unclear whether the referrer either failed to spot the pathology or did not review the x-ray at all," the CQC report said.
"Ten months later the patient was re-x-rayed and found to have advanced spread of lung cancer."
Inspectors added: "There was a reliance on the referrer to interpret their patients' x-rays. The delay in diagnosis caused significant harm to both patients."
A third case is still under investigation.
Inspectors found that between April 1 2016 and March 31 2017, 26,345 chest x-rays and 2,167 abdomen x-rays had not been formally reviewed by a radiologist or an appropriately trained clinician.
The 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."
Dr Nicola Strickland, president of the Royal College of Radiologists, said: "Patient x-rays must be viewed as quickly as possible by a radiologist or appropriately trained clinician.
"That the Queen Alexandra Hospital encouraged staff who were not suitably trained to review them, in an attempt to manage its sheer volume of unreported scans, is a concern."
Mark Cubbon, chief executive of Portsmouth Hospital NHS Trust, said: "We have issued an unreserved apology to the families of the three patients who experienced harm because of the delays to their care.
"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.
"Any delays to patient care are totally unacceptable. We take the CQC's concerns extremely seriously and fully accept the findings of the inspection report highlighting delays in reporting chest x-rays.
"When these issues were raised with us in the summer, we immediately put in place a range of improvements to address the concerns highlighted by the CQC."