Pre-teens are "ill-equipped" for the "avalanche of pressure" on social media as they head into secondary school, the Children's Commissioner for England has warned.
Anne Longfield said for many children the social media world begins to dominate their social life, with many craving and seeing likes and comments as a way of validating their identity.
In a report published by the commissioner on Thursday, called Life In Likes, it states that children become increasingly anxious about their online image as they begin Year 7.
Speaking to the Press Association, Ms Longfield said it has been known for a while that more and more children under 13 are using social media apps, despite not being designed for their use.
The study, involving eight groups with 32 children aged eight to 12, found the most popular social media platforms are Snapchat, Instagram, Musical.ly and WhatsApp.
But Ms Longfield said the most surprising discovery for her was the change in how social media is used between the ages of nine and 10, and up to 12 and 13.
"What starts as fun usage of apps - children are using it with family and friends and to play games when they are in primary school - turns into an avalanche of pressure when children really are faced with a cliff edge of social media interaction when they start secondary school," she said.
This move is centred around a social pressure to be constantly contactable and connected, with children describing this as an important expectation of their friendships, and fallouts over not being responsive enough, the report said.
Some Year 7 children described how receiving notifications from across the social media platforms, especially if there were a number of them, was distracting, time consuming and stressful to manage.
Describing the start of secondary school as a "very pressured time for children anyway", the commissioner said it is likely that by the age of 11-12 most are likely to have a smartphone.
Pressed on whether social media use can be detrimental, Ms Longfield said it provides a way of children passing judgment on the popularity of others and what they look like, and can be "very negative for children".
"We know it is hugely damaging for children in terms of their self identity, in terms of their confidence, but also in terms of their ability to develop themselves as individuals," she said.
"So they are ill-equipped when they enter secondary school, and we'd like schools, and parents and social media companies to help them prepare for what that means emotionally."
The report noted how children demonstrated a strong understanding of the importance of being themselves online, but how these attitudes did not always translate to their actual behaviour.
Older children in the groups often talked about wanting to look like those they see online, and an increasing anxiety over whether their posts would be liked.
"They want to look like the popular people online, and we see that that increases as they start to follow celebrities," she added.
"Then there is this push to connect - if you go offline will you miss something, will you miss out, will you show that you don't care about those people you are following, all of those come together in a huge way at once.
"For children it is very, very difficult to cope with emotionally."
Calling on schools and parents to prepare children for this change in social media use towards the end of primary school, the commissioner also warned it needs to be dealt with "swiftly".
"It is something which if we don't deal with now, it will mean that it grows over time and children will have greater problems as they move through secondary school," she added.
Stating how schools have done an awful lot in addressing safety online, Ms Longfield also called for digital literacy and online resilience lessons for Year 6 and Year 7 students, so they learn about the emotional side of social media.
With many social media platforms having an over-13 age limit, the commissioner said it is clear companies are "still not doing enough" to stop younger children using them.
Quizzed on whether social media companies also have a responsibility, she said they "really do" because "ultimately these apps weren't designed for children".
A Government spokesman said they expect social media firms to have robust processes in place around the use of their sites by children.
"As part of our Internet Safety Strategy, we'll consider whether companies need to do more to close down accounts belonging to under-age children and be more transparent in their messaging," he added.