According to the child counselling service, a total of 16,644 counselling sessions were given to children with mental health concerns between 23 March, the start of lockdown, and 10 May.
The NSPCC, which runs the support line, said this was more than half of the total 30,868 sessions delivered during the seven-week period.
This equates to around 339 sessions a day, or roughly one every five minutes.
In comparison, over the 2018-19 financial year, a total of 71,283 counselling sessions were given to children with mental health concerns in the UK.
This means the service has delivered 43 per cent of last year's total mental health counselling sessions within just seven weeks.
The children’s charity adds that 36 per cent of sessions during the lockdown mentioned concerns about mental or emotional health, 13 per cent touched on suicidal thoughts and feelings, 12 per cent on family relationships, six per cent on self-harm, and four per cent on sex, relationships and puberty.
Furthermore, around 120 sessions mentioned coronavirus on average each day, with 5,880 sessions – around a fifth of the total – taking place during lockdown.
Between 10 January, the first time a child mentioned coronavirus when contacting Childline, and 10 May, there have been 6,938 sessions delivered which mentioned coronavirus.
The NSPCC also said use of Childline's Calm Zone, an online tool to help young people manage their feelings, had increased fourfold during lockdown.
In early March, it was being accessed around 2,400 times a week, rising to 10,000 weekly page views in April.
Now, the charity is urging the public to donate to its urgent appeal, Still Here for Children, launched in April so it can continue to provide support to young people.
Dame Esther Rantzen, who founded Childline, said that children and families have faced “extraordinary challenges” because of the coronavirus pandemic, and warned that the next few months will be no different.
She added that Childline provides a “vital lifeline” to young people who are trying to cope with the fear, anxiety and distress caused by the pandemic.
“We know from the counselling sessions we have delivered that children's mental health has been directly impacted and those children who are living in homes which are not safe - where there is violence, addiction or abuse - are finding it especially difficult during lockdown,” Rantzen explained.
”Young people in times of stress often find that their friends and the security of school are crucial to their mental wellbeing, but now they are without them, and are isolated and alone.
“It is imperative that we are there, particularly for those whose usual support networks are not in place and have nowhere else to turn.”
Childline said there has been little change in concerns pre and post-lockdown, but that the crisis has exacerbated young people's worries and circumstances.
These include can include abuse, domestic violence and difficult family relationships.
An 18-year-old girl told Childline: ”I am scared about the Coronavirus. I'm finding it hard to cope at the moment and it feels like it's the end of the world.
“When I was little I developed OCD and had been to therapy which has helped me to deal with the stress better.
“However, the Coronavirus has triggered it again and I'm really struggling, I don't know what to do. Please help.”
In March, the counselling service reported a spike in the number of calls it was receiving from distressed young people struggling to deal with the pandemic.
Almost two-thirds (597) of Childline’s sessions took place between 16 and 22 March, while the peak was 18 March, when prime minister Boris Johnson announced school closures. There were 121 calls that day.
If you have been affected by this article, you can contact Childline by calling for free on 0800 1111. Alternatively, you can log in for a one-to-one counsellor chat on the Childline website or send an email.
Any adult who is worried about a child can contact the NSPCC helpline on 0808 800 5000, or by emailing email@example.com.