The study, commissioned by online information firm Garlik, found that 23% of children aged between eight and 12 claim to use the three most popular social networking sites - Facebook, Bebo and MySpace - despite the minimum age registration being 13 (or 14 for MySpace).
Additionally, over a quarter (26%) of children aged between eight and 15 years old admit they have strangers as friends on their social networking page, and a fifth (20%) have met up with strangers they have only ever encountered online, according to the report.
The research revealed that this has led to parents taking matters into their own hands to protect their children, with 72% admitting that they snoop on their kids' online activities.
One in four (25%) parents secretly log into their child's social networking page, while a quarter (26%) have set up their own social networking page to spy on their children.
However, nearly all of the parents (89%) have spoken to their children about the dangers posed by social networking sites.
Tom Ilube, Garlik chief executive, said: "That parents feel compelled to monitor their children on this scale should send a powerful message to the big social networking sites.
"With three quarters of a million underage users in the UK, Facebook, MySpace and Bebo need to take their own age restriction policies far more seriously to help allay parents' real fears."
Garlik commissioned online teenage community Dubit to conduct the research among 1000 children aged 8-15 years in June 2008. Market research firm PCP conducted research among a sample of 1030 UK parents.