And they must do more to monitor their use of sites such as Bebo and Facebook, watchdog Ofcom said.

Ofcom published research showing that 27% of eight to 11-year-olds, who have internet access, bypass online age restrictions to put their profiles online. The report also found that many parents and children lack awareness of the issues surrounding privacy and safety on the internet.

Since they first appeared in the late 1990s, social networking sites have flourished with the launch of Friends Reunited in 2000 and more recently Bebo, MySpace, Facebook and a Saga site for over-50s.

Ofcom said the websites - many of which are based in the US - could do more to remind people of the risks involved in putting all their details online.

The research found that, of those with internet access, almost half (49%) of eight to 17-year-olds and just over a fifth (22%) of adults now have a profile on a social networking site.

But 16% of parents do not know who can see their children's profiles and many believe that their children are safer online than they actually are.

While 41% of children admitted that they did not use privacy settings, only 30% of parents knew that their children's profiles were open to view by anyone online.

Robin Blake, head of media literacy at Ofcom, which commissioned the research, said: "When their children go out to play, parents often state ground rules about when they come back and when to meet etc. Parents need to recognise that going online has the same kind of risks."

But he said there were "huge benefits" for children using the social networking sites, as long as they did so carefully, adding: "We don't want parents to pull the plugs out of the wall."