Two-fifths of people who have an abnormal bowel test result and are referred for further investigation decide to drop out of the screening process, a study has found.
Experts are anxious to find out why so many of these patients ignore their next screening invitation two years later.
Two possibilities are that either they do not think further screening tests are necessary after undergoing other procedures or they are put off by their experiences.
Just 13% of those who had a normal result opted not to continue with screening, according to the study of almost 40,000 men and women living outside London in the south-east of England.
In England, everyone aged 60 to 69 is invited to take part in the bowel cancer screening programme, which involves looking for hidden traces of blood in stool samples. The age limit is being extended to 74.
Eligible participants are sent a home-testing kit every two years. People who receive an abnormal result from the test, called the Faecal Occult Blood test (FOBt), are referred for follow-up procedures including a colonoscopy to rule out cancer.
Previous research has shown that up to 12% of those asked to have follow-up tests do not turn up for their appointments.
The new study found that almost 40% of this group also fail to partake in further screening.
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Even after getting the all-clear from an initial colonoscopy, they could be putting themselves at risk by dropping out, according to the experts.
Lead scientist Dr Siu Hing Lo, from the Health Behaviour Research Centre at University College London, said: "Our research has identified a small but high risk group who are failing to continue with bowel cancer screening tests.
"We urgently need to understand why people are dropping out of bowel cancer screening and not attending the follow-up investigations as we know the test saves lives."
Bowel cancer accounts for 13% of all cancers diagnosed in the UK. Each year the disease affects more than 41,000 new patients in the UK and claims about 16,000 lives.
The findings appear in the British Journal of Cancer, published by Cancer Research UK.
Julie Sharp, head of health information at the charity, said: "Bowel cancer is the fourth most common cancer and screening is important to help us beat the disease.
"Only 58% of people who are offered bowel screening in England complete their testing kits and it's a concern to see that people who have abnormal results are dropping out of the screening programme."
Father-of-two John Marsh, 67, from London, who was diagnosed with bowel cancer in 2007, said: "It was thanks to the bowel cancer screening programme that my cancer was found at an early stage.
"I'd had no symptoms at all but a month after my 60th birthday, I got a screening test in the post.
"I'd never heard of it, but I did it straight away. I didn't have a second thought about doing it.
"After doing the test I got a letter asking me to do the test again and then I was called for a colonoscopy.
"I saw the consultant and he confirmed it was bowel cancer. I had surgery in the March but I didn't need chemotherapy. I've had three colonoscopies since then and they've all been clear.
"I know a few people who haven't done it and I just can't understand it."