A rising number of patients are facing long waits for key NHS tests as UK hospitals struggle with staff shortages, a BBC investigation shows.
Tests such as scans and biopsies are crucial for diagnosing illnesses.
But tens of thousands of patients are waiting weeks - sometimes months - for them, leading to potentially harmful delays before treatment can start.
Figures obtained by the BBC under the Freedom of Information Act suggest a lack of radiographers is a key cause.
They carry out scans and ultrasounds which can detect problems such as tumours, heart disease and multiple sclerosis.
But the data provided by hospitals show one in 11 posts are unfilled.
In total, 124 NHS trusts and boards across the UK - four in five of those asked - responded to the BBC's request for information.
The figures provided showed there were 14,067 funded posts in April, but 1,283 were vacant.
The Society of Radiographers said the shortages were causing significant delays in diagnosing illnesses.
Currently there are more than one million patients waiting for tests, such as MRI and CT scans, ultrasounds and endoscopies across the UK.
In England and Scotland they are meant to have them within six weeks of referral.
But the number of patients waiting longer than that has risen by 9,000 in England to nearly 29,000 in the past year. One in seven has waited more than three months.
The performance in Scotland and Northern Ireland is even worse.
In Scotland the numbers waiting more than six weeks have jumped by more than a third to more than 18,500.
Northern Ireland has set a target of nine weeks, but has seen the numbers waiting longer than that rise by nearly a fifth in the past year to more than 56,000. Some 22,000 of these have waited more than six months.
Only in Wales has the situation improved.
The findings come only a month after Prime Minister Theresa May said early diagnosis of cancer was going to be a key focus of the forthcoming long-term NHS plan in England.
In her speech to the Conservative Party conference, she said she wanted to see three-quarters of cancers diagnosed early by 2028 - currently only half are.
Richard Evans, head of the Society of Radiographers, said he was concerned about the situation.
"If we are going to identify things like cancer early we need more diagnostics.
"We are struggling to cope with demand and that creates delays for patients.
"It is not just about staffing either. We have ageing machines that are not as efficient as they should be."
Emma Greenwood, Cancer Research UK director of policy, was urgently needed.
"The diagnostic bottleneck is already threatening the care of many and presents a challenge which must be faced head on if we are serious about improving care for cancer patients."
But the Department of Health and Social Care said plans were already in place to increase the number of radiographers and invest in "state-of-the-art technology".
Health Education England's Cancer Workforce Plan published last year set out plans to recruit an extra 300 diagnostic radiographers, while university applications for courses are rising, a spokesman said.
"Cancer is a priority for this government, and we are radically transforming how we screen, diagnose and treat the disease," he added.
A spokesman for the Scottish government said investment was also being made to reduce diagnostic waiting times as part of the £4 million Radiology Transformation Programme.
Freedom of Information research by Patrick Cowling