This study makes the massive assumption that removing the growths would save lives

Dr Wendy Atkin, Imperial Cancer Research Fund

However, although it is a relatively simple procedure, it only examines the lower, or distal, part of the colon.

To look further requires a colonoscopy, which uses a different probe, and needs far greater skill on the part of the doctor carrying it out.

The research, published in the New England Journal of Medicine, looked at 1,564 people aged over 50 screened with colonoscopy as part of an occupational health scheme at work.

Fifty of these had advanced growths - with potential to turn cancerous - in their upper colon.

However, 23 of these had no growths in the lower part of the colon - meaning a sigmoidoscopy might well have given them the all clear.

Dr Daniel Podolsky, from Massachussets General Hospital, said that relying on sigmoidoscopy to screen for colon cancer was like giving women mammograms on just one breast to check for breast cancer.

He criticised the failure of US medical insurance companies to pay out for colonoscopy testing.

However, many doctors believe that colonoscopy, for clinical and practical reasons, is a poor choice of test for screening people who appear perfectly healthy.

Safety first

A key argument is safety - patients who have colonoscopy run a far higher risk of major complications, such as the probe perforating the bowel wall.

It is ethically unjustifiable, say some, to make hundreds of thousands of 50 to 60-year-olds run that risk every year to pick up a very small number of extra cancers and abnormal growths.

The number of otherwise healthy patients harmed by the procedure might end up outnumbering the number whose lives were saved.

Dr Wendy Atkin, deputy director of the Imperial Cancer Research Fund's Colorectal Cancer Unit at St Mark's Hospital in Harrow, said that there was no evidence that removing abnormal growths actually protected the patient from cancer.

She said: "This study makes the massive assumption that removing the growths would save lives.

"Flexible sigmoidoscopy takes five minutes to do, is more convenient and comfortable for the patient and we think that nurses can do it."

Practical impossibility

She said that the skills required to safely carry out colonoscopy meant it was a practical impossibility to offer it to every healthy adults.

Waiting lists for colonoscopy - even for those with clear symptoms that need investigating - run into months in some parts of the NHS.

Bowel cancer is a disease in which early detection can radically improve the chances of survival. By the time symptoms become obvious, the cancer has often grown deeply into the wall of the colon.

Doctors are still undecided whether it would be best to organise a screening programme for everyone, or test only those who report with suspicious symptoms, such as blood in the faeces, weight loss and other gut problems.

If universal screening is to be carried out, there are other methods which have been suggested, including the faecal occult blood test, in which the patient simply provides a stool sample which is checked for invisible traces of blood.