THE world's five billion mobile phone users may be at risk of certain types of brain cancer and have been urged to consider ways of reducing their exposure after new research.

The World Health Organisation’s (WHO) International Agency for Research on Cancer (IARC) said a review of all scientific evidence suggested mobile phone use should be classified as “possibly carcinogenic”.

Other items in this category include lead, engine exhaust fumes and chloroform.

It is the furthest any report has gone in warning about the effects of mobile phone radio waves on the human body.

The study, carried out by a working group of 31 scientists from 14 countries, contradicts an earlier statement by the organisation that there were no established links between the handsets and cancer.

Jonathan Samet, IARC chairman, said: “After reviewing the evidence ... the working group classified radio frequency electromagnetic fields as possibly carcinogenic to humans.”

He added some evidence suggested a link between an increased risk of glioma, a type of brain cancer, and mobile phone use.

However, cancer experts urged the public to keep the findings in perspective.

Ed Yong, head of health information at Cancer Research UK, said: “The WHO’s verdict means there is some evidence linking mobile phones to cancer but it is too weak to draw strong conclusions from. The vast majority of existing studies have not found a link between phones and cancer, and if such a link exists, it is unlikely to be a large one.

“The risk of brain cancer is similar in people who use mobile phones compared to those who don’t, and rates of this cancer have not gone up in recent years despite a dramatic rise in phone use during the 1980s. However, not enough is known to totally rule out a risk, and there has been little research on the long-term effects of using phones.”

The decision comes after a study last year, which looked at 13,000 mobile phone users over 10 years, found no clear answer on whether the devices cause brain tumours.

The new findings could prompt the UN health body to look again at its guidelines on mobile phones, the IARC scientists said, but more research is needed before a definitive answer on a link can be given.

The report has been keenly awaited by phone companies and campaign groups who have raised concerns about whether mobile phones might be harmful to health. Around five billion handsets are currently in use worldwide, with 76 million handsets owned in the UK.

The WHO said a person using a mobile phone about 12 inches away from their body, for example when text messaging, accessing the internet, or using a hands free device – will have a much lower exposure to radio-frequency fields than someone holding the handset against their head.

Further advice states that exposure is also reduced by limiting the number and length of calls. Using the phone in areas of good reception also decreases exposure as it allows the phone to transmit at reduced power.

Commercial devices for reducing radio frequency field exposure have not been shown to be effective, WHO officials said.