More than a decade's use of mobile phones does not increase the risk of brain cancer, according to one of the largest studies yet conducted into the link.

The finding is reassuring with long-term use of mobiles increasing. Worldwide users of mobile phones now number more than one billion and concerns have grown about the possible health effects of the devices.

But the latest study of 420,000 users who had owned mobiles for up to 21 years has confirmed earlier findings that there is no evidence of risk.

Mobiles emit radio waves that can penetrate four to six cms into the human brain, raising fears that they may trigger malignant changes in brain cells.

The latest study published in the US Journal of the National Cancer Institute, is a follow up to a study in 2001. It extends to an average of eight and a half years the period during which mobile users were monitored, with a maximum of 21 years.

Professor Tricia McKinney, Centre for Epidemiology and Biostatistics, University of Leeds, said: "The large numbers of subscribers in the study mean we can have some confidence in the results that have not linked mobile use to a risk of any cancer, including brain tumours."