The consumers association, Which? says there's little evidence to back clinics' claims that paying through the nose for a medical 'MOT' is key for future health.

The association asked two experts in public health to assess screening services offered by the top three private health providers and two independent private clinics.

They concluded that while screening tests sometimes detect future problems, and sometimes people will benefit from early treatment, the danger lies with those that give inconsistent results and little useful information, and those that 'detect' non-existent disease.

The majority of people who get positive results from health screens wouldn't actually go on to develop the disease. False results like this cause worry, more investigations, and even unnecessary treatment - at a cost to the individual or the NHS.

There's no evidence, for example, that routine screening for prostate specific antigen (the main test for prostate cancer) leads to a longer or better quality of life. Two thirds of men with high PSA don't have prostate cancer, and some men with prostate cancer don't have high PSA.

Private clinics criticised

The Diagnostic Clinic in London and The London Clinic were strongly criticised for providing inadequate information. The Diagnostic Clinic didn't give a balanced explanation of the huge range of conventional and complementary tests on offer, or tell patients there aren't any recognised quality standards for many of these tests. Several have actually been proven unreliable, such as a test that uses an electro-dermal device banned in the US because of a lack of scientific evidence.

Information provided by The London Clinic about the likely benefits, harms and limitations of its tests was, again, 'totally inadequate', says the report. There's no evidence to support its screening procedures, and it was sketchy about how tests can actually reduce the risk of disease.

It states, for example, that the pelvic examination is 'looking for liver/ovarian or colon disease'. In fact, evidence shows this is an inappropriate method of finding such diseases.

Bupa was the best of the bunch for customer information, said to be 'as good as the NHS.' But again, there was little strong evidence to back up the tests' medical benefits, while others are available on the NHS. One expert said the only advantages of going to Bupa were pleasant surroundings and the fact you'd have more of the doctor's time than during a normal visit to a GP.

It was a similar story at BMI; client information was judged 'very misleading', while the claims for the screening were 'very vague.' For example, it ignored the limitations for mammography in women under 50. Nuffield Hospitals offered the genuine benefit of a GP helpline, which customers can access for a year after screening. Otherwise, though, the experts thought that only some of the tests would be of benefit.

Malcolm Coles, editor, Which?, says: "In some cases, screens can have a positive impact and provide reassurance, but our experts had major misgivings about the value of paying for full-body screens. Whatever the test, it's important that enough information and support is given before, during and after testing, but some of the clinics we looked at are giving inadequate or misleading information."

Dr Muir Gray, Programme Director for the NHS Screening Committee, says: "The NHS offers various free screens and can also give advice and support on various health-related issues. A major benefit of NHS screening is that you receive full continuity of care - including follow-up treatment."