Official guidance for millions of people who use networking sites will be published today by the information commissioner amid growing concerns that young people are being naive about the personal details they put online.
The watchdog fears that most youngsters do not realise that the information they place on websites such as MySpace and Bebo leaves an electronic footprint which could be traceable to them in the future. In a survey by the Information Commissioner's Office almost 60% of young people aged 14-21 said they did not realise the information they placed online could be permanently linked to them.
David Smith, ICO deputy commissioner, said: "The cost to a person's future can be very high if something undesirable is found by the increasing number of education institutions and employers using the internet as a tool to vet potential students or employees."
Despite the lack of awareness about how long information can remain online, most youngsters said they were keen to retain their privacy: seven in 10 surveyed said they did not want any potential employers looking at their profiles on social networking sites.