The strong suggestion is that they were aware of possible problems.

Simon Best, Editor, Electromagnetic Hazard and Therapy

While this has not been scientifically verified, says the patent application, the "uncertainty" is affecting the "speed of growth" of the mobile market.

The documents emerge at a time when mobile phone companies are facing legal actions by people claiming their health has been damaged by mobiles.

Law firms in the UK have also been approached by former mobile users who want to sue.

Simon Best, editor of a newsletter called Electromagnetic Hazard and Therapy, which features articles discussing a possible threat to health from mobiles, says that the companies may be preparing in case they are forced to provide such protective devices with every new mobile.

He told BBC News Online: "The strong suggestion is that they were aware of possible problems.

The balance of significant evidence to date suggest that mobiles - operating within international exposure guidelines - don't cause adverse health effects

Spokesman, Federation of the Electronics Industry

"Legally, they are in a cleft stick over this one - they have been denying there is a problem for so long, yet at the same time they have been developing these."

A spokesman for Nokia said: "Patenting innovations is standard practice for all aspects of technology within the industry and antennas are no exception to this rule.

"Nokia is continuously striving to improve all aspects of its product design."

A spokesman for Motorola said: "The safety of our customers is of paramount importance.

"The innovations in the patents address the design, performance and efficiency of our products and were not motivated by concerns about potential health issues."

A spokesman for the Federation of the Electronics Industry, which speaks on behalf on mobile phone companies in the UK, said that there was no proven link between mobile phones and ill health.

"The balance of scientific evidence to date suggest that mobiles - operating within international exposure guidelines - don't cause adverse heatlh effects."

Major studies into the issue are underway across the world, with the UK government launching a �7.3m probe - part funded by the industry.

A recent British Medical Association report suggested that there is no evidence that mobile use is cancer-promoting.

However, the official government advice is still precautionary, with parents advised to minimise the amount of time their children spend talking on mobiles, as their developing brains are thought to be more vulnerable.