The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday April 20 2005

We were wrong to say, as we did in the article below, that everyone over the age of 55 is to be offered a colonoscopy. Instead, the Department of Health plans to start offering people in their 60s a test to detect blood in stools from April next year. This programme is expected to reduce deaths from bowel cancer by 15% in the population invited for screening.

Within the next year, as part of an initiative to reduce bowel cancer, the government will embark on a programme to offer colonoscopies to millions of people. But if that sounds like a new cervical smear-type programme, think again. Smears are a simple, five-minute, in-and-out procedure. Colonoscopies, if my experience is anything to go by, are anything but simple and straightforward: the people invited to have a colonoscopy are in for a big shock.

The plan to introduce a massive screening operation was announced by health secretary John Reid in October. He said it would target everyone in the country aged over 55: the plan is to reduce the number of bowel cancer deaths by at least 15%. Bowel cancer is a serious problem. It currently affects around 35,000 people a year, and will kill 20,000. Testing and early detection are the keys to saving lives.

Unfortunately, testing is not a pleasant procedure. I happen to have a condition called ulcerative colitis, which means that I have an increased risk of getting bowel cancer. As a result, over the past eight years I have had three colonoscopies: none of them was much fun.

About two weeks before the procedure, the hospital sends me a pack containing a diet sheet, directions to the hospital and a bag of Picolax, a powerful lemon-flavoured laxative. For four days before the procedure I have to eat what is described as a "residue free" diet. This means no beans, pulses or brown rice and no fibrous vegetables, which could make it harder for the hospital staff to see what's going on in my bowel. Instead, I have lots of white fish, chicken breast and white rice.

On the day before the procedure I am supposed to eat nothing at all. In practice this means I subsist on boiled sweets and chicken stock, which won't show up in a colonoscopy examination. Twice during the day I have to take a sachet of Picolax, mixed into a glass of water. As a result, the floodgates of my bowels are well and truly opened and I am ready for inspection.

The following day I am not allowed to eat or drink anything at all - not even a cup of tea. Then comes the examination itself, which always reminds me of some 1960s British science-fiction film. I start off in a ward partitioned with old chintz curtains where I sign a consent form acknowledging that the procedure might leave me doubly incontinent or even dead, and I then walk into what looks like a space station. It is full of doctors and nurses with white coats, looking at enormous television monitors.

I sit on a bench and a needle is put into the back of my hand. A nurse then injects me with a mixture of the painkiller pethadine - the same stuff that is given to women in labour. And there is also a sedative called midazolam. With a bit of luck that's all I can remember. On one occasion, though, they woke me up from the procedure so that I could turn around - what I recall is having terrible stomach cramps while they displayed a picture of the contents of my bowel on the screen.

Laura Doig, nurse specialist in endoscopy at Guys and St Thomas's Hospital in London, has performed about 3,000 of these procedures. What happens, she explains, is a colonoscope - a one and a half-metre-long tube, with a bundle of fibre optics and a videochip so operators can see the bowel lining - is pushed up the back passage. Tools can also be put into the tube, so samples of the wall (biopsies) can be taken. It can also be used to burn off any pre-cancerous collections of cells (polyps) that are found. Air and water can be blown down the tube to give a better view.

What that means is that, when I wake up again, it's in a room full of people breaking wind in their sleep. My wife comes to collect me and we go for something to eat - I'm starving by this point - and for the rest of the day I'm not allowed to drive or operate heavy machinery. Which is just as well, because my behaviour is erratic. I feel woozy, have trouble walking in a straight line and can become euphoric.

And this is the experience that, if the government gets its way, everyone over the age of 55 will soon have. In readiness, seven regional and three national endoscopy training centres have been established, and the department of health says that within the past few months, it has provided training for 345 new endoscopists and 88 new trainers.

But will this be enough to carry out the large number of endoscopies that are needed? Mike Thompson, consultant colorectal surgeon at the Queen Alexandra Hospital, Portsmouth, believes that it will be impossible to provide this sort of service for five years. "It's a difficult thing to do. You need people who can do it gently. Unless you are skilled, you can't get an endoscope all the way around the colon."

There is also a problem of compliance. Most healthy people don't want to go to hospital, get knocked out and have a camera pushed up them. In fact, says Thompson, it's difficult to get more than 55% of people to go through this.

Coloproctologists in America approve of the "colonoscopies for all" approach, but Thompson believes there are better alternatives.

He points out that there are risks with colonoscopy - around one in 1,000 patients will suffer a bowel perforation, and indeed this week's British Medical Journal carries a report of a 47-year-old woman who developed life-threatening complications after the procedure. Those who have a polyp removed are at particular risk, and one in five of those whose bowel is perforated will die as a result. "It's too high a level of risk for routine screening," Thompson says.

Other systems may be more successful. The government is piloting a system in which people post a small sample of stool to a testing centre. If blood is detected, then the person is referred for a full colonoscopy. The only drawback of this is that the bleeding only occurs once a cancer is present: the test cannot detect the precancerous stage. The test is cheap but it has to be repeated every two years.

Perhaps the best hope lies with a technique called flexible sphygmoidoscopy. A small, flexible device, about 60cm (23in) long, is used. This does not require the patient to follow a special diet, takes only a few minutes and doesn't involve much pain. It is also very cheap. Although it can only look at the left side of the colon, this is where most cancers occur. The procedure only needs to be done once: doctors generally reckon that if a person has not developed polyps by their mid-fifties, they are unlikely to get them at all. A full study of this technique is being undertaken and initial results are promising.

Most experts believe that the biggest barrier to the wider uptake of colonoscopies is cultural. In Britain people are sensitive about bowel problems. "A lot of people have hang-ups about anything to do with the bum," says Thompson. "Unless they can deal with this, it's difficult for us to help them."