The regulator of heath care services in England is calling for a limit on how long it takes to report the results of patients' x-rays and scans.
The Care Quality Commission's review of radiology practices found that the process could vary from one hour to two working days in hospital A&Es.
This could mean delays in results being shared with doctors and patients receiving timely care.
The review also said there were not enough radiologists to meet demand.
The Care Quality Commission asked all 151 health trusts in England to reveal their own internal benchmarks for radiology reporting - and it found huge variations in the time taken to examine and report on scans and x-rays.
- For urgent or fast track cases, this varied from two to five working days
- In A&E, the range was one hour to two working days
- And for GP referrals, while some trusts aimed to report back within 24 hours, others settled for 21 days
The review also said a shortage of radiologists was contributing to delays and backlogs.
The CQC has called for national standards for hospitals to ensure timely reporting.
Cancer Research UK said radiology scans were crucial in diagnosing many cancers, and it was extremely worrying that some patients were waiting a long time.
'Disparity in timescales'
Serious concerns had already been aired after inspections of three NHS trusts, where backlogs and delays to radiology reporting were felt to be putting patients at risk.
The trusts were Worcester Royal Hospital, Kettering General Hospital and Queen Alexandra Hospital, Portsmouth.
Professor Ted Baker, CQC's chief inspector of hospitals, said: "While our review found some examples of good practice, it also revealed a major disparity in timescales for interpreting and reporting on examinations, meaning that some patients are waiting far longer than others for their results.
"We are calling for agreed national standards to ensure consistent, timely reporting of radiological examinations," he said.
"This will allow trusts to monitor and benchmark their own performances - and ensure that, for example, patients are not put at risk by delays in their x-ray results being reported to the clinician responsible for their care."
He added that many trusts relied on outsourcing to external providers or delegating reporting to non-radiology clinical staff to cope with an increasing workload.
But he said that those with the job of interpreting x-rays had to be "appropriately trained to do so".