Parents have been issued warnings over the dangers of giving their children a mobile phone. As a survey revealed that 91 per cent of 12-year-olds now have a mobile, the Health Protection Agency called for parents to limit their use.

"If there are risks - and we think that there could be - then the people who are going to be most affected are children," said Dr Jill Meara of the Health Protection Agency. "We would call on parents to make sure they examine all the evidence before deciding if they will give their children a phone.

"They do have a benefit but nothing in life is without some hazards, and you need to weigh the benefits against the potential risks. We will not know for several years what the real risks of mobile phones are."

There are fears that the electromagnetic radiation emitted from handsets may harm health. In particular, there have been claims that it could affect the body's cells, brain or immune system and increase the risk of developing a range of diseases from cancer to Alzheimer's.

The government advises a " precautionary approach" and Dr Meara warned children to limit the time they spend on the phone.

She said: "If you're going to have an hour-long chat with your boyfriend, do it on a landline. It's also worth sending a text instead if you can, as that cuts the amount of time you are exposed to radiation."

Mobile phone companies today claimed that increased use has helped children communicate more effectively.

"The mobile has become the most important electronic device for young people in the UK today,," said Charles Dunstone, chief executive officer of The Carphone Warehouse Group, which commissioned the study. "It provides them with a social network, a sense of security and access to entertainment. But most importantly it provides them with a sense of belonging to their peer group."

The HPA called for government leaflets outlining the potential risk of mobile phones to be given to every buyer.

"These leaflets were available a few years ago, but seem to have disappeared," said Dr Meara. "They really need an update."

The study found that just a quarter of 11 to 17-year-olds are worried that their mobile phone might harm their health, compared to one third of parents.

Most young people also admitted they were concerned their phone put them at risk from muggers. The Mobile Operators Association, which represents all the UK's mobile networks, said the decision on whether to have a phone was up to parents.

"Mobile technology offers reassurance to parents and children who value being able to stay in touch with one another," said a spokesman. "Parents can weigh these benefits against health concerns.

"All mobile phones sold in the UK comply with international health and safety exposure guidelines adopted by the European Council of Health Ministers in 1999."