The Department of Trade and Industry released its long-awaited report on the Effectiveness of Mobile Phone Radiation Shields on Friday.

The report, commissioned by the DTI in response to the Stewart Report, released in May 2000, was carried out by an independent science team, which found that mobile phone shields do reduce radiation but other devices clearly do not.

"Keeping the device as far away from the operator's head reduces the risk of radiation, but the results clearly indicate that it does not stop it entirely and indeed different devices offer different levels of protection," said a spokesperson at the Department of Health.

But earlier this year the US Federal Trade Commission brought actions against two companies selling mobile phone shields — which claimed their devices cut 99 percent of harmful radiation — on the grounds that this could not be proved.

"The problem is more legal than scientific. Companies cannot advertise goods as doing a job, unless when called upon to do so they can prove their claims. Obviously in this case the US felt they could not," said a spokesman at the Office of Fair Trading.

The DTI's report showed that some shields did reduce radiation by as much as 98 percent and in some cases as little as 18 percent. But they also sap the phone's battery life by an average of 50 percent.

"The reductions are unidirectional and the small amounts of absorbing material are not particularly effective," said the report. "Without specific understanding of how phones work, it is possible that much is taken on trust without reliance on any sound physical basis for the benefits claimed."

But absorbing devices known as 'buttons' did nothing to reduce SAR (specific absorption rate) levels.

Hands-free kits, which separate the user's head from the phone, were welcomed by the report's authors. But the study highlighted that, in order to avoid exposure to other parts of the body, it is essential that the phone be kept a certain distance away.

The government is advising users to follow the Department of Health's guidelines.

A full copy of the DTI's report can be found via its home page.