Calls from under-11s wanting support for loneliness have soared by almost three quarters in the past five years, Childline said.
The counselling service also said it is contacted 15 times a day on average by children struggling with loneliness.
The helpline, run by the NSPCC, said it delivered 5,564 counselling sessions with children on this issue between April 2021 and March last year.
It added that it had a 71% increase in calls from under 11s seeking support for loneliness compared to 2017/2018 figures.
In 2021/2022 Childline delivered 327 counselling sessions on loneliness to children aged 11 and under, up from 191 in 2017/2018.
The NSPCC said it has launched a campaign called Day In The Lonely, partnering with supermarket Lidl, to let children struggling with loneliness know that such feelings do not need to last.
The charity said it aims to encourage young people, with the support of their parents, to speak about their mental health issues early on, so it does not escalate to crisis point.
Shaun Friel, Childline director, said: “Loneliness is sadly an issue that a high number of children and young people are experiencing, and, at Childline, we know the impact this can have on a child’s wellbeing and mental health.
“We’ve had cases of children left feeling depressed, that’s why we are reminding all children that talking to a trusted adult like a parent, teacher or Childline counsellor about being lonely is a brave thing to do and it will enable them to get the help they need.
“Through this campaign, we want to raise awareness amongst children and adults, that there are many reasons why young people might feel lonely, but these feelings don’t have to last and that getting support early on can prevent things from escalating in the future.”