The NSPCC said it delivered 4,063 counselling sessions via its Childline service to under-18s in 2016/17, who said they were struggling with feelings of isolation. This was the first year the organisation collected data about the problem, after noticing a rise in calls related to the issue.
Female callers made at least 73 per cent of the calls (2,978), compared with at least 14 per cent made by boys (582 calls). The remaining callers (503) did not disclose their gender.
NSPCC chief executive Peter Wanless said there was no single reason why so many children were experiencing isolation. Some callers blamed social media for leading them to make unrealistic life comparisons with other people, which left them feeling "ugly and unpopular".
Others said they felt lonely after moving house or school, or losing a close friend or relative after a death or relationship break-up.
"What is clear is that the world is becoming an increasingly complex place to grow up in with children and teenagers' facing daily pressures to achieve what society defines as a successful life - grades, relationships, physical appearance," Wanless said.
"It is therefore vital that children and teenagers have people around them, in particular parents, who they can really open up to about how they are feeling."
Those affected said they often spent time alone in their bedrooms, which made them feel worse. In some cases callers had self-harmed or contemplated ending their own lives.
One 15-year-old girl told Childline: "I've thought about ending my life because I think it's pointless me being here. I don't feel like anyone cares about me and I'm lonely all the time."
Commenting on the findings, National Council of YMCAs' chief executive Denise Hatton said it was important for all young people to be able to take part in activities that would help "tackle the root" of loneliness.
"Local YMCAs work with 230,000 young people a year and we know that youth services, such as youth clubs, give young people the opportunity to feel part of their community," she said.
"Yet over the years these services have been a frequent and easy target for local authorities looking to make savings in reduced budgets.
"In this digital age where family dynamics and friendships have changed dramatically, opportunities for young people to take part in activities and socialise offline are now more important than ever."
In December 2016, Childline reported it had also seen a rise in the number of children calling for help with gender identity concerns.