A radiation expert is calling for more research into the long-term health effects of using mobile phones.
The government warns against excessive mobile use
Lawrence Challis, chairman of the Mobile Telecommunications Health Research Programme (MTHR), said it was "responsible" to study long-term users.
Professor Challis is seeking Department of Health funding to study 200,000 mobile phone users over a decade.
He said there was a "hint of something" suggesting a link between mobiles and ill health but no hard evidence.
Prof Challis, a physicist, said short-term studies had established no risk but added that volunteers should be looked at over a period of at least five years.
He is negotiating with the Department of Health and the mobile phone industry - which jointly fund the MTHR - for £3m extra to carry out more research.
Prof Challis told BBC Radio 4's Today programme: "The situation at the moment is that we have no evidence of any harm from mobile phones.
"The dilemma is the time that it takes for a disease to appear.
"We know from smoking and with the bomb falling in Hiroshima that nothing was seen for 10 years."
He added: "The responsible thing to do is to continue monitoring."
Most of the volunteers taking part in the study will have used mobile phones for about 10 years, Professor Challis said.
Research published last year suggested no evidence that mobile phone users had a higher risk of tumours in the brain, eye, or salivary gland, or leukaemia.
The Danish Institute of Cancer Epidemiology in Copenhagen, looked at data on more than 56,000 people who had been using the devices for at least 10 years.
But Prof Challis advised caution, saying: "I certainly don't want my grandchildren to start using mobile phones until they are at secondary school.
"Children may be more vulnerable, so we need to protect them.
"My balanced view is keep them off of them until they get to secondary school but encourage them to text as much as possible. That gives them much less exposure."
The government advises mobile phone users - of which there are more than one billion worldwide - to keep their call times short.