A pause on non-urgent treatment, a shortage of radiographers and a reduction in people coming forward for tests are said to have affected scans across England.
Teams at Surrey and Sussex Healthcare NHS Trust performed more than 10,000 fewer imaging scans between March last year and February in comparison to the same period in 2019-20, NHS figures show.
Imaging tests that can be used to diagnose or detect cancer were carried out 55,510 times during that time - a 15 per cent drop.
The number of brain MRI scans fell by nine per cent while there were 16 per cent fewer chest x-rays performed and a drop of 19 per cent in ultrasounds of the pelvis and abdomen.
Across England the number of scans used to diagnose or discount cancer dropped by a quarter to around 8.5 million as lockdown measures gripped the health service.
Postponements of non-urgent healthcare throughout spring last year contributed to the steep decline in diagnostic imaging activity levels.
But a reduction in people coming forward for tests has also had an impact, according to an NHS spokesman, while the Royal College of Radiologists say the crisis was exacerbated by a shortage of radiographers that predates the pandemic.
Dr William Ramsden from the RCR said waiting lists for hospital treatments were at a record high, with many patients on those lists likely to require scans.
Calling for investment in radiologist training, he added: “The pause in non-elective NHS work during the coronavirus peak last spring had a huge knock-on impact on waits for scans and treatment.
“NHS England has helped increase radiology capacity, coming through with emergency funding for scanners and private provider support, which must continue."
Michael Wilson CBE, chief executive at Surrey and Sussex Healthcare NHS Trust added: “Over the past year we have seen fewer referrals for scans across our cancer services even though our services continued to run during this period.
"We are now seeing an increase in patient referrals, and our teams are working incredibly hard to ensure that anyone being referred is seen as quickly as possible.
"Early diagnosis is really important, so we encourage anyone who is worried about their symptoms to see their GP.”
Dr Ramsden said the RCR supported NHS plans to speed up scan access via diagnostic hubs but said thousands more radiologists were needed in order to ensure the changes could be implemented successfully and patients treated, scanned and diagnosed in a timely manner.
An NHS spokesperson praised the tireless efforts of NHS staff and said diagnostic imaging services had continued despite the impact of the pandemic, with 90 per cent of urgent referrals with suspected cancer seen within two weeks in February.
She said: “We know that fewer people have come forward for checks during the pandemic but we continue to urge anyone experiencing worrying symptoms that could be cancer to contact their GP as soon as possible so the NHS can investigate them as soon as possible.”