Paying for a private health check may seem a good idea but it could do you more harm than good, a study suggests.
Which? questioned the usefulness of some of the tests
Two public health experts assessed screening services offered by the top three private health providers and two independent health clinics, for Which?
Some tests gave inconsistent results and some detected non-existent diseases - which could cause unnecessary worry and investigations, the study said.
The private health providers disputed the study's conclusions as "wrong".
People in the UK spend about £65m a year on screening tests.
While some can detect future problems and allow early treatment, others can provide little useful information, says Which?.
For example, an abnormally high PSA (prostate specific antigen) test result could mean a man has prostate cancer, but more often it is caused by something less serious like an inflamed prostate or an enlargement of the prostate that often comes with ageing.
The PSA test is not a foolproof test for prostate cancer.
For every 100 men with a raised PSA level, only about 30 of them turn out to have any cancer cells in their prostate.
It is also true that occasionally the PSA may read "normal" when there really is cancer there.
This is why the NHS does not advocate routine PSA screening, and only offers the test to men who wish to be tested and understand the uncertainties of any result.
The Which? report criticised two independent private clinics in London - The Diagnostic Clinic and The London Clinic - for providing inadequate information to consumers.
According to the report, The Diagnostic Clinic did not give a balanced explanation of the huge range of conventional and complementary tests on offer, or tell patients there were not any recognised quality standards for many of the tests.
Similarly, it said information provided by The London Clinic about the likely benefits, harms and limitations of its tests was "totally inadequate".
"There's no evidence to support its screening procedures and it was sketchy about how tests can actually reduce the risk of disease," says the report.
"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," the authors say.
BUPA was the best of the top three private health providers the experts looked at for customer information.
But it was criticised for having little strong evidence to back up tests' medical benefits. Some of the tests it offered were free on the NHS.
Which? editor Malcolm Coles said: "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.
"Some of the clinics we looked at are giving inadequate or misleading information," he said
Dr Muir Gray, programme director for the NHS Screening Committee, said: "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 and treatment," he said.
The London Clinic disputed the outcome of the report.
"The public health doctors could not have reviewed the brochure in depth," said a spokesman.
He added: "Had the offer [of a health screen] been taken up, the Which? researcher would have received appropriate information pertaining to the benefits, risks and outcomes of each test."
Dr Rajendra Sharma, medical director at The Diagnostic Clinic said: "There has always been criticism of alternative techniques, but we at The Diagnostic Clinic believe that these tests, when used in conjunction with conventional tests, provides a unique insight into our patient's health."
Dr Peter Mace, clinical director of BUPA Wellness said: "Which? is wrong.
"Almost 100,000 people a year have a BUPA health assessment: 96% of them are satisfied with the quality of service, 95% of them feel reassured after a BUPA health assessment - even where a medical problem has been detected - and three quarters of them make a lifestyle change as a result."