A mass study of the long-term impact of mobile phones is to be undertaken amid fears that people who have used them for more than ten years are at greater risk from brain cancer.

More than 200,000 volunteers, including long-term users, are to be monitored for at least five years to plot mobile phone use against any serious diseases they develop, including cancer and Parkinson's and Alzheimer's diseases.

Professor Lawrie Challis, who is in the final stages of negotiation with the Department of Health and the mobile phone industry for the £3 million that he needs to fund the study, told The Times that research has shown that mobiles are very safe in the short term but that there is a "hint of something" for people using them longer.

In an interview, Professor Challis, a world expert on mobile phone radiation, and chairman of the government-funded mobile telecommunications health research programme, emphasised that the "hint" was just that. One European study has found a slight association and using a mobile for more than ten years. The few long-term users developed more acoustic neuroma brain tumours which were found close to the ear used for phoning.

But, because of the tiny numbers involved, "it could be by chance," he said. Asked whether the mobile phone could turn out to be the cigarette of the 21st century in terms of the damage it could inflict, he replied: "Absolutely."

He said that the study was necessary because all the important breakthroughs in what caused cancers had shown that the effects often took more than ten years to show. "You find absolutely nothing for ten years and then after that it starts to grow dramatically. It goes up ten times. You look at what happened after the atomic bombs at Nagasaki, Hiroshima. You find again a long delay, nothing for ten years. The same for asbestos."

He made plain that he was not put off because many existing studies had shown no dangers. "The fact that you don't see anything in ten years is also more or less what you would expect if there is something happening," he said.

Announcing the new study, he said: "Because there is a hint and because the professional epidemiologists who I trust and who do this all the time feel there is a chance that this could be real, they can't rule out the possibility. And because we all know that most cancers don't show up for more than ten years, I think you have to carry on. It's essential we carry on.

"Otherwise what are we going to do? If in ten or fifteen years' time people start getting trouble it won't show up until it's a really big effect."

The move was welcomed by the Conservatives. Andrew Lansley, the Shadow Health Secretary, said: "It's not scare-mongering to ask these questions for future generations. At the moment there is little evidence to suggest that use of mobile phones has any impact on health, but it is vital that there is continuing research to establish if long-term use is a danger."

Professor Challis is planning a separate study monitoring the impact of mobile phone use on children. He disagreed with the claim of some scientists that there was no cause to believe mobiles affected them differently from adults.

"We all know that if you're exposed to sunlight as a kid you are much more likely to get skin cancer than if you're exposed as an adult."

He insisted that there was nothing irresponsibly alarmist about his message. Even if a risk were found, people would not have to stop using phones, but perhaps reduce their use."I do it because I think it's worthwhile," he said.

A science of ifs, buts and maybes . . .

2006 Largest study yet, of 420,000 Danish users for up to 21 years, ruled out any large effect on any cancer after short or long-term use

# It suggested there could be a very slightly raised risk of acoustic neuroma, a rare, benign cancer of the inner ear, among users of more than ten years

# A raised risk on the side on which sufferers said they used their phones was balanced by a decreased risk on the other side - which led the scientists to suggest recall bias as the likely explanation. They said no firm conclusions could be drawn

2006 US study suggested lower sperm counts among heavy users. It is widely thought that this reflects another aspect of heavy users' lifestyles, such as stress or a lack of exercise

2005 Interphone international study finds no effect on acoustic neuroma for ten years of use. It was unable, however, to rule out an effect for longer-term use, because of insufficient data

2004 Study suggests users have a higher risk of brain cancer if they live in rural areas. It has been suggested that this could reflect the higher strength of signal in areas with few base stations

2003 Swedish study suggests higher risk of acoustic neuroma among heavy users of analogue mobile phones, which have since been phased out. Scientists criticised the methodology of the research

2002 Finnish research suggests that phone emissions can cause abnormalities in blood vessel cells in the laboratory. Scientists that said it was not possible to draw conclusions for phone safety for real people