Long-term use of mobile phones may cause cancer, government scientists have admitted, as the biggest review ever of the subject is published.
The report, by a Health Protection Agency group set up to examine the safety of mobile phones, transmitter masts and wi-fi, found "no convincing evidence" they caused any adverse effects on human health.
But members of the Advisory Group on Non-ionising Radiation (AGNIR) said they could not be sure of the long term effects of mobiles, as there was currently "little information beyond 15 years from first exposure".
A small number of individual studies have found evidence of a link between heavy mobile phone use and increased brain tumour incidence.
Two years ago the INTERPHONE study reported that the heaviest users could be at a 40 per cent increased risk of developing glioma, a common type of brain cancer. Most studies have found no such association though.
However, brain tumours can take decades to develop. Launching the 333 page report, which reviews hundreds of studies, group chairman Professor Anthony Swerdlow said: "I think there's a need to keep a watch on national cancer trends in relation to this, particularly with brain tumours.
"So far brain tumour rates are not rising in the sorts of age groups who have had exposure for 10, 15 years.
"But if this is something that takes 15, 20 years or more to show up ... we need to keep watch over rates just in case."
Researchers running cohort studies - projects following individuals' health over their lifetimes - also needed to investigate the matter to see if heavy users of mobile phones tended to develop brain tumours more than others, said Prof Swerdlow, an epidemiologist at the Institute of Cancer Reseach,.
The review found no evidence that radiofrequency (RF) electromagnetic fields caused by wi-fi - now widespread in schools, homes and the workplace - caused harmful effects, or could even be detected by those who claimed to be sensitive to it.