In the trial, many of those who blame masts for their symptoms reported greater distress when they thought the signal was on, suggesting the problem has a psychological basis.

"Belief is a very powerful thing," said Professor Elaine Fox, of the University of Essex, who led the three-year study.

"If you really believe something is going to do you some harm, it will."

The study was funded by the Mobile Telecommunications and Health Research programme, a body which is itself funded by industry and government.

Modern appliances

It is unclear how many people in the UK suffer from "electro-sensitivity", an allergy they believe can be triggered by a range of modern day appliances from hair driers to mobile phone masts.

But the HPA research did not consider the effects of waves from phone masts, as most of the studies looking at electrical sensitivity were carried out before they were widely introduced.

A number of studies subsequently have looked at the mobile effect, but the Essex experiments are some of the largest and most detailed to date.

After 12 of the sufferers dropped out of the trial, researchers tested a total of 44 people with a history of symptoms against a control group of 114 people who had never reported ill effects from masts.

When the signal was being emitted, and they were told of this, sensitive individuals reported lower levels of well-being.

This was true for exposure to both forms of mobile systems - GSM and UMTS (3G).

However, when tests were carried out in which neither the experimenter or participant knew if the mast was on or off, the number of symptoms reported was not related to whether a signal was being emitted or not.

Chance finding

Two of the 44 sensitive individuals correctly judged if it was on or off in all six tests, as did five out of 114 control participants.

"This proportion is what is expected by chance," the researchers said.

The symptoms were real. As well as reporting feeling unwell, sensitive individuals had sweatier skin and higher blood pressure - both measures of a physiological response.

But this was regardless of whether the signal was on or off.

"Hence the range of symptoms and physiological response does not appear to be related to the presence of either GSM or 3G signals," the study concluded.

Other experts endorsed the study's findings.

Dr James Rubin, of the Mobile Phone Research Unit, King's College London, said the findings were in line with those from most other previous experiments.

"This should be reassuring news for anyone who is concerned about the possible short-term health effects of masts," he said.

But Mast Sanity declared "history has shown that many now commonly accepted physical conditions were initially dismissed as psychological".

"Isn't it time that the government woke up to the reality of electrosensitivity instead of attempting to persuade sufferers that it is all in their minds?" said spokeswoman Yasmin Skelt.

But another campaign group, Powerwatch, commended the research as "one of the best designed and executed studies to date" while stressing that the number of dropouts was unfortunate.

"So whilst it cannot be entirely ruled out that a small minority are truly sensitive, the proportions of any truly sensitive people are likely to be far lower than the 3-35% that has been quoted."