A third of children aged between five and seven have a social media profile despite being under the minimum age requirement, a new study suggests.
Analysis from Ofcom shows that TikTok is the most popular platform among the youngest users, who have been nicknamed TikTots.
Britain's communications watchdog found 16% of children in the UK aged three and four are already using the platform.
It warned that some children could be concealing aspects of their online activities from their parents using "finstas" or fake Instagram accounts, with 64% of eight to 11-year-olds having multiple profiles, and 46% of these having an account just for their family to see.
More than a third of children reported engaging in what the regulator 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%), while 19% deleted their browsing history, and 6% circumvented parental controls put in place to stop them visiting certain apps and sites, it said.
The regulator called on tech firms to protect users from misinformation.
"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," said Melanie Dawes, Ofcom CEO.
"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."
Ofcom's annual study also found that over a third of internet users in the UK are unaware that online content or account could be false or biased.
30% of UK adults, 14.5 million, who use the internet are unsure about or do not consider the truthfulness of the information they see online. A further 6%, or about one in every 20 internet users, trust everything they see online.
The regulator found that both adults and children overestimate their ability to spot misinformation.
It warned that the "sheer volume" of information meant having the critical skills and understanding to decipher fact from fiction had "never been more important".
Respondents were shown social media posts and profiles to determine whether they could verify their authenticity. Although 69% said they were confident in identifying misinformation, just 22% were able to correctly identify the tell-tale signs of a genuine post, without making mistakes.
There was a similar pattern among older children aged between 12 to 17, with 74% confident but only 11% able to identify genuine content.
Similarly, 24% of adults and 27% of children who claimed to be confident in spotting false information were unable to identify a fake social media profile in practice.
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