One in five teenagers has met someone face-to-face whom they first encountered on the internet, according to research into the risks taken by young people online.

The study found that teenagers also freely hand out personal information to strangers. Details divulged include full name (30 per cent), address (12 per cent), mobile number (20 per cent) and where they go to school (46 per cent), while 9 per cent had posted family photos.

Other surveys have found the number of teenagers meeting up offline ranges from 7 per cent to 14 per cent. Experts said that the growth in social networking sites such as MySpace could account for the higher figure in the more up-to-date research by Garick, the online security company, in March.

The study found that parents are often ignorant about what their children are up to, with only a fraction aware that they are befriending strangers.

Although nine out of ten parents questioned said that they monitored their children’s online activities, more than half the teenagers questioned admitted to going online without their parents’ knowledge, usually late at night.

John Carr, the Government’s adviser on online safety for children, said that girls were particularly at risk. “The most vulnerable group are girls between the ages of 12 and 15 who are going through puberty and becoming interested in boys and relationships,” he said. “You only need to listen to the news to see how common it is for girls to be lured into meetings.”

Most recently Jade Powell, a 14-year-old girl from Chelten-ham, went missing with an 18-year-old man she met on the internet. She was found safe and well five days later.

Another study, conducted last year by Webwise, found that 27 per cent of teenagers had encountered someone online who asked for personal information, such as a telephone number, while 23 per cent had received unwanted sexual advances.

Tom Ilube, chief executive of Garlik, urged parents to hammer home lessons on internet safety.

“Our research is a shocking wake-up call to all parents in the UK to sit down with their children and talk about how to keep safe online,” he said. “The Web is a wonderful place to explore but young people continue to make themselves vulnerable by not applying the same caution online as they would in person.”

Mr Carr said that parents were in a difficult position. “If they go on too much about it, or make out that the internet is completely unsafe, their children will just not listen.

“But it is part of modern parenting to teach children about the dangers of the internet, in the same way as teaching children the facts of life or to be honest.”

However, unlike the facts of life, parents frequently know little about the internet and feel ill-equipped to tackle the subject. To this end the Government has sponsored a site for parents, thinkyouknow.co.uk, which has useful advice and links to other relevant sites.

‘I have no idea what websites he visits’

Nicola Ejedewe says that she has no idea what her son Josh, 16, is up to as he spends hours in the sitting room on the family computer.

“It’s a huge world out there and I find it terrifying that I don’t really know,” she said. “Up until a few years ago I felt I had good parental control.

Now I don’t.” Mrs Ejedewe, 37 of North London, takes every opportunity to discuss safety with her son. “But I don’t have the knowledge to track what he has been doing and what sites he goes on,” she said.

Josh said: “My mum has a big thing about downloading sites because we have downloaded viruses in the past and then the computer has to be fixed. She has set up passwords for the music sites I use and only she has the password, so she has to be there for me to go on them.”

Josh chats to others online. “I have met people online from round here and it has been nice to chat to them. But I’d never agree to meet them.”

Times recommends

• Are eldest children really a cut above?
• Peaches Geldof: censored
• Top 10 style-setting films of all time