Parents have to monitor their children's use of the internet because the rules set by social networking sites are too easy to ignore, new research shows.

According to the recent study by online protection firm Garlik, there are over 750,000 underage children logging into the social networking sites of Facebook, Myspace and Bebo.

The sites' terms and conditions state that children aged between eight and 12 should not register for networks, but because the rules are not enforced, nearly three quarters (72 per cent) of parents take it upon themselves monitor their children's site usage.

A quarter of parents were even found to have set up their own social networking page to spy on their kids.

"Facebook, Myspace and Bebo need to take their own age restriction policies far more seriously to help allay parents' real fears," said Tom Illube, Garlik chief executive.

"That parents feel compelled to monitor their children on this scale should send a powerful message to the big social networking sites."

Another recent survey conducted by YouGov by BroadbandChoices.co.uk, found the majority of parents have verbal agreements with their children to stop them accessing certain sites.