Prostate cancer is the most common cancer in men in Britain, killing around 10,800 men last year, but is relatively difficult to diagnose early due to its subtle symptoms and a reluctance among many men to visit a doctor.
Advocacy group Cancer Research UK said University of Cambridge scientists compared the grading of more than 800 men's cancers before and after they had surgery to remove their prostate and found that many had more advanced cancer than initially thought.
Of the 800, 415 patients' prostate cancer had previously been classified as slow growing and confined to just the prostate.
However, around half these cases turned out to be more aggressive, and a third had cancers that had spread beyond the prostate.
"This highlights the urgent need for better tests to define how aggressive a prostate cancer is from the outset, building on diagnostic tests like MRI scans, and new biopsy techniques which help to more accurately define the extent of the prostate cancer," said Greg Shaw, a urological surgeon and one of the study's authors.