Data analysed by the Royal College of Psychiatrists shows that almost 400,000 children and 2.2 million adults sought help for mental health problems during the pandemic, with 1.68 million more mental health sessions delivered over the past year compared with the year before.
The head of the college is warning that the demand for services is reaching breaking point, with a "very real risk" they will be "overrun by the sheer volume of people needing help".
And the chief executive of a Leeds mental health charity says it is "critical" that young people's mental health needs are dealt with now, as "it could be many months or even years before we fully recognise the pandemic’s toll on our collective wellbeing".
Analysis of NHS Digital data suggests that while the Covid-19 crisis is affecting people of all ages, the under-18s are suffering most, the college said.
Some 80,226 more children and young people were referred to specialist mental health services between April and December last year, up by 28 per cent on the same months in 2019 to 372,438.
Meanwhile, 600,628 more treatment sessions were given to children and young people, up by a fifth on 2019 to 3.58 million.
Overall, 18,269 children and young people needed urgent or emergency crisis care, including assessments to see if they needed to be sectioned under mental health laws.
This is an increase of 18 per cent on the figures for 2019 and includes assessment for illnesses such as anorexia and other eating disorders.
The Royal College of Psychiatrists is calling for an additional £500 million in the Government's mental health recovery plan to urgently reach the front line.
This is on top of the existing planned investment in mental health services set out in the NHS Long Term Plan.
Dr Bernadka Dubicka, chairwoman of the child and adolescent faculty at the Royal College of Psychiatrists, said: "Our children and young people are bearing the brunt of the mental health crisis caused by the pandemic and are at risk of lifelong mental illness.
"As a frontline psychiatrist I've seen the devastating effect that school closures, disrupted friendships and the uncertainty caused by the pandemic have had on the mental health of our children and young people.
"Services were already struggling to cope with the number of children needing help before the pandemic hit, and they risk being overrun unless the Government ensures the promised money reaches the front line quickly."
Helen Kemp, Chief Executive of Leeds Mind, said the trend was evident in local services in the city. Between April and August 2020 the charity's reception staff noted an increase in enquiries around young people’s mental health, mostly from worried parents or guardians.
She said its young people’s peer support group THRU (Talk, Relate, Help, Understand) is currently oversubscribed and many of its mental health workshops have been fully booked.
Ms Kemp said: "There is clearly a demand for mental health support in our young people locally.
"Over the past year, our young people’s peer support groups have often focused on anxieties around the pandemic – whether that was about social isolation, loneliness, disruption to education, or more recently, about the vaccine.
"There have also been lots of other big issues for young people to contend with as the rest of us have, for example, around racism or unemployment.
"We’ve also been seeing more interest from schools and other education institutions around the types of mental health support that we can offer to the young people in their learning community.
"No doubt this will be fuelled by Government confirming earlier this week that £79m of the £500m package for mental health services would be allocated to Children and Young People.
"While the additional detail and the package as a whole is clearly welcome, it could be many months or even years before we fully recognise the pandemic’s toll on our collective wellbeing. It is critical that we respond to children and young people’s mental health requirements now."
The data also show that adults are suffering, with more than one million additional treatment sessions given to adults between April and December last year (1,078,539), an increase of 8% on 2019.
Some 159,347 urgent or emergency crisis referrals were made for adults - an all-time high - and an increase of 2% on 2019.
Dr Adrian James, president of the Royal College of Psychiatrists, said: "The extent of the mental health crisis is terrifying, but it will likely get a lot worse before it gets better.
"Services are at a very real risk of being overrun by the sheer volume of people needing help with their mental illness.
"While the recent funding announcement is welcome, we need this money to reach mental health services as soon as possible to tackle this crisis."
Barnardo's chief executive Javed Khan said: "The coronavirus pandemic has felt like a lifetime for children and young people, and the negative effects could last a real lifetime if they do not have the right mental health support.
"We owe it to all children struggling with their mental health to use the pandemic as a catalyst for change and build a system that is radically better than the one we had before.
"That means moving towards an inclusive approach to children's mental health and wellbeing that embraces holistic support in schools and the community, digital services and alternative therapies alongside clinical care - with children and young people having their say in how these services are run."
Health minister Nadine Dorries said: "I am acutely aware of how difficult this pandemic has been for many, especially children and young people, and I remain absolutely committed to supporting the mental wellbeing of everyone.
"Early intervention and treatment is vital, and we are providing an extra £2.3 billion a year to mental health services, this will help an additional 345,000 children and young people access NHS-funded services or school and college-based support by 2023/24.
"Last month we announced a cross-government mental health recovery action plan, backed by an additional £500 million, specifically targeting those that have been most impacted by the pandemic including those with severe mental illness, young people, and frontline staff."