More than 200,000 patients are being forced to wait more than a month for radiology test results, including to diagnose cancer, a royal college has warned.
A report into NHS services in England by the Royal College of Radiologists (RCR) found three-quarters of hospitals have a backlog for radiology, which includes CT and MRI scans.
Some 126 out of 155 NHS trusts responded to enquiries from the RCR about the state of radiology services.
The report found 230,000 patients on one snapshot day in February had been waiting more than a month for test results. These included at least 12,000 who were waiting for the result of CT or MRI scans, which are often used to detect or monitor cancer.
The study said: "NHS patients are feeling the effects of a chronic failure to invest in radiology services. The cost of this failure is enormous and will continue to grow until it is fixed."
The RCR said patients were left feeling anxious while they waited for results, on top of the health effects of delaying a diagnosis of cancer or other conditions.
It argued that NHS money was being "wasted" on outsourcing work to the private sector which it said should be spent on growing the NHS workforce.
The survey found NHS trusts are spending increasing amounts on "costly and inefficient outsourcing".
Spending on extra help for radiology rose from £47 million to £73.8 million between 2014 and 2015. The RCR said this would pay for around 900 extra NHS radiologists.
Giles Maskell, president of the RCR, said: "Early diagnosis of serious medical conditions such as cancer is vital so that patients have the best chance of cure. Any delay caused by the shortage of radiologists can lead to worse outcomes for patients.
"The impact of these delays is felt not only in the potential for delayed diagnoses but also in the anxiety felt by patients, their families and friends.
"Without immediate and sustained investment to address the chronic shortfall in radiologist numbers, the ambitions for earlier diagnosis of cancer set out in the 2015 English Cancer Strategy will simply not be realised."
A 2014 census from the RCR found 88% of radiology departments in the UK were unable to fulfil their requirements around producing reports on scans due mainly to the shortage of radiologists.
There were also 338 unfilled radiology posts in England, the census found.