Patients are facing delays in diagnosis because of severe shortages among pathology staff, according to a report seen by the BBC.
A survey by the Royal College of Pathologists found only 3% of the NHS histopathology departments that responded had enough staff.
Histopathologists are doctors and scientists who diagnose and study diseases such as cancer.
Hundreds more pathologists are now working in the NHS, health chiefs said.
The new report by the Royal College of Pathologists says that demand for pathology services has grown significantly in recent years, but staffing has not increased at the same rate.
It carried out a workforce survey of histopathology departments throughout the UK in 2017.
Of the 158 departments, 103 responded.
Only 3% said they had enough staff to meet current clinical demand.
And 45% of departments had to outsource work while half of departments were forced to use locums.
“The cost of staff shortages across histopathology departments is high for both patients and for our health services.
“For patients, it means worrying delays in diagnosis and treatment,” said Prof Jo Martin, president of the Royal College of Pathologists.
“We estimate the cost of locums and outsourcing work is £27m each year across the UK health service, money that could be better invested in staff and new diagnostic equipment” she added.