Bowel cancer has become a leading killer in the UK, where there are around 30,000 new cases each year.
An audit by the British Society of Gastroenterology found that up to a third of all people who undergo a colonscopy examination are not examined fully.
A colonscopy, carried out with fibreoptic tube, is used to search for growths, or polyps, in the large intestine.
Dr Roger Leicester, chairman of the BSG's audit committee, said lack of training and supervision was to blame.
He said: "The audit is disappointing, but not surprising.
"Endoscopy (colonoscopy is a form of endoscopy) is just over 30 years old. It was never a separate specialism so people did not have proper training in it.
"In terms of skills, it is the most difficult endoscopy procedure to perform."
Detailed study
The report, published at a BSG meeting in Glasgow, is the world's most detailed audit ever into colonoscopy.
The findings include:
Patient discomfort and technical failure were also given as reasons for incomplete examinations.
But Dr Leicester said this was probably due to poor training.
Dr Leicester urged people not to be concerned that they had been misdiagnosed.
He said: "It would be a very small percentage of people who were misdiagnosed."
He revealed that 70% of growths occurred on the left side of the colon, but that the audit revealed that it was the right side of the colon that was mostly likely to be missed.
In 90% of cases the left side was examined.
Training scheme
Dr Leicester said a unique training scheme have been set up to make sure the early signs of cancer are spotted.
He added that a 100% success rate was being achieved on the courses and welcomed the £2.5 million allocated by the Government over the next three years to improve training.
The report was conducted in East Anglia, North-West Thames and West Midlands health authorities.
The BSG worked with Royal College of Physicians, Royal College of Surgeons, Association of Coloproctology and the Association of Surgeons to collect data from 600 patients, 8,900 procedures, 230 colonoscopists and 42 endoscopy units.