A survey of 100 oncologists found that 53% said they often or sometimes prescribe treatment that is not necessarily the best choice for a patient because of a lack of access to genetic tests.
In addition, 40% of respondents said they often or sometimes prescribe a cancer treatment that is not necessarily the most appropriate because of delays in test results.
Genetic testing allows oncologists to decide which agents are most likely to be effective at treating particular tumours.
Dr Tim Iveson, consultant medical oncologist at Southampton General Hospital and member of Bowel Cancer UK's medical board, said the findings were 'extremely worrying'.
'It is simply not acceptable that some patients are not getting the treatment that they deserve,' he said.
Over 70% of respondents to the survey said they thought cost was the main barrier to wider roll-out of genetic testing. The survey was sponsored by Merck Serono, which has agreed to fund NHS genetic testing for UK bowel cancer patients.