A survey by the magazine Health Which? looked at four different services costing �20 to �105.

Five researchers, three with known allergies, tried out each testing service twice.

They found that the tests could not "reliably diagnose true food allergies or intolerances".

They added that in cases where known allergies were not identified the tests could prove "positively dangerous".

However, expets at the York Nutritional Laboratory dismissed the report, questioning its accuracy and claiming it did nothing to clarify issues for consumers.

The York Laboratory, which has carried out food intolerance tests for more than 26,000 patients over 16 years, said there was a clear difference between classical allergies and intolerances that was dismissed or not fully appreciated by the authors of the Which? report.

John Graham, of YNL, explained that classical allergies were characterised by a rapid onset of symptoms which were usually sudden and dramatic making the offending substance relatively easy to identify and often only one food was involved.

But in the case of food intolerances the reactions were usually delayed over hours or even days after ingestion of offending foods.

Also more than one food was normally involved and the foods were extremely difficult to identify without laboratory testing.

Mr Graham said that a clear majority of these patients with chronic illnesses found symptom relief when the problem foods were identified and removed from their diet.

In a six-month survey 72% of respondents claimed a significant improvement to their health within 60 days.

He added: "Obviously we were unaware that Health Which? were assessing us in this way but if we had been we could have explained to them from the outset so they could have avoided the confusion they have created.

"There can be no doubt that this current misrepresentation is a missed opportunity not just for the publication but for its readers who are being misled and whose health may well be improved through food intolerance testing."

Sue Freeman, from Health Which?, stood by the report's findings and said: "We are concerned that consumers are being taken advantage of by these expensive allergy testing services. Certainly anyone relying on them to identify a classic allergy could be putting themselves in danger.