A third of children aged between five and seven have a social media profile despite being under the minimum age requirement, according to a new Ofcom report.
TikTok is the most popular platform among the youngest users, who have been nicknamed TikTots, with 16% of children in the UK aged three and four already using the platform.
Ofcom has also warned that some children could be concealing aspects of their online lives from their parents using "finstas" or fake Instagram accounts, and is calling on the tech companies to ensure users are protected from misinformation.
It comes ahead of parliamentary debates about the government’s new Online Safety Bill and the expected appointment of Conservative Party peer Michael Grade as Ofcom’s new chair.
According to Ofcom, the communications regulator, two thirds of children aged eight to 11 have multiple accounts or profiles, with almost half having an account just for their family to see.
More than a third of children reported engaging in what Ofcom described as potentially risky behaviours as they could hinder a parent keeping an eye on the child’s internet use.
"A fifth surfed in incognito mode (21%), or deleted their browsing history (19%), and one in 20 circumvented parental controls put in place to stop them visiting certain apps and sites (6%)," the regulator said.
The majority of teenagers say they feel positive about the benefits of being online and use the platforms as a force for good.
Over half (53%) say using social media was good for their mental health, while only 17% said it was not.
A quarter of teenagers said they used the internet to find out about healthy eating or get help with "growing-up issues" like relationships and puberty.
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Ofcom released the figures as it called on tech companies to protect their users, warning that more than a third of those users "are unaware that online content might be false or biased".
"Every minute sees 500 hours of content uploaded to YouTube, 5,000 videos viewed on TikTok and 695,000 stories shared on Instagram," the regulator said.
This flood of information means the ability to think critically about the truthfulness of online information has "never been more important", according to Ofcom, which found that 6% of adults in the UK believe everything they see online.
Dame Melanie Dawes, its chief executive, said: "In a volatile and unpredictable world, it’s essential that everyone has the tools and confidence to separate fact and fiction online – whether it’s about money, health, world events or other people.
"But many adults and children are struggling to spot what might be fake. So we’re calling on tech firms to prioritise rooting out harmful misinformation, before we take on our new role helping to tackle the problem."