Private health tests for diseases such as cancer and other possible medical problems could do more harm than good, an investigation claimed today.

The Consumers Association magazine Which? said that screening services, which cost Britons around £65m a year, offered poor value for money. Many tests had no scientific basis.

The magazine warned that some tests were causing unnecessary worry, and clinics were not giving patients enough information to help them understand the results.

Which? asked two screening experts to assess the information and tests provided by five "full-body screening services".

It said that many tests gave widely inconsistent results and little useful information. Some screens also "detected" non-existent diseases, causing unnecessary worry and potentially costly and invasive investigations. The report also said that breast screening for women under the age of 50 could lead to misleading results and unnecessary surgery.

The experts strongly criticised two of the clinics - the Diagnostic Clinic in London and The London Clinic - for providing inadequate information. They said the Diagnostic Clinic did not give a balanced explanation of the huge range of conventional and complementary tests it offered, or tell patients there were not any recognised quality standards for many of the tests.

The experts also branded the information given by The London Clinic about the likely benefits, harms and limitations of its tests as "totally inadequate".

Bupa came out best for its customer information, with the experts saying it was "as good as the NHS".

But they added that many of the tests offered lacked strong evidence of real benefits, with others available on the NHS anyway.

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."

Dr Rajendra Sharma, medical director at the Diagnostic Clinic, said: "There has always been criticism of alternative techniques, but we believe that these tests, when used in conjunction with conventional tests, provide a unique insight into our patient's health.

The London Clinic said it disputed the report. "Patient information on the benefits, risks and outcomes for each of the health screening tests is freely available and would have been provided to Which? had Which? adequately undertaken their research," the clinic said in a statement.

It said the Which? report was based on two doctors reviewing their marketing brochure.