A quarter of the 10.8 million Britons registered with such sites admitted leaving details such as their telephone number and date of birth on their profile pages, said Internet security group GetSafeOnline.org.

Those aged 18-24 were found to be even more at risk, with more than a third (34 percent) leaving confidential information online.

The findings came from a poll of 2,000 people carried out by ICM for the group.

"Although some of these details may seem harmless, they actually provide rich pickings for criminals," said Tony Neate, managing director of GetSafeOnline.org.

"Your date of birth and where you live is enough for someone to set up a credit card in your name.

"So whilst most people wouldn't give this information to a stranger in real life, they will happily post it online where people they don't know can see it."

The poll also found that 15 percent of people did not use any privacy settings on social networking sites and nearly a quarter (24 percent) used the same password for all Web sites.

And 29 percent of respondents admitted to using social networking sites to look up former girl- and boyfriends. The figure was even higher for 25- to 34-year-olds, at 39 percent.

Nearly one in three people had also used the sites to find out about their boss, colleagues or a job candidate.

© 2007 Agence France-Presse