There is a "vast gap" in the support provided by the NHS for children with mental health problems, the Children's Commissioner in England says.
Anne Longfield's report criticised slow progress made in improving specialist community services for children.
She said waiting times were too long and was concerned about numbers being rejected by services in some areas.
It comes as the NHS prepares to publish a major report into mental health problems among the young.
The figures will be released by NHS Digital on Thursday, after its statisticians were asked by the government to investigate the scale of the challenge.
The last time this was done – in 2005 – around 10% of children were found to be struggling with a mental health problem.
But the Children's Commissioner expects that figure to rise.
What has the Children's Commissioner found?
The commissioner looked at specialist community mental health services provided by the NHS known as CAHMS.
Ms Longfield said there was a "vast gap" between what was needed and what was provided.
An analysis of NHS figures from 2017-18 show that 325,000 children were treated – fewer than 3% of the population.
The figures showed that more than a third of people referred to the services were not accepted.
This could be because their needs were not severe enough to need help and could be dealt with through other services, such as in school or by councils.
But Ms Longfield said she was concerned children were getting turned away because services simply did not have time to see them.
Her report also raised concerns about waiting times. Just under half of people who received treatment after a referral in 2017-18 had waited longer than six weeks. The average waiting time was nearly two months.
What needs to happen?
The commissioner believes children's services are under-funded. Around £700m is spent on CAHMS and eating disorders support. By comparison, services for adults receive 15 times more despite children representing 20% of the population.
The commissioner said an extra £1.7bn would need to be invested to bring children's services in line.
She said this could help pay for more early help by funding NHS counsellors in schools for example.
Emma Thomas, chief executive of Young Minds, said the commissioner was right to highlight the problem.
She said the charity gets "calls every day" about children who are waiting for help or have been denied help.
"This can have devastating consequences – in some cases, children start to self-harm, become suicidal or drop out of school while waiting for the help they need."
She agreed early intervention and better funding were essential.
What is the government doing?
Both NHS England and the Department of Health and Social Care have made improving children's mental health care a priority.
In fact, the commissioner's report noted that investment was increasing and there had been good progress in terms of tackling eating disorders with new services and strict targets for access.
Last month, in his Budget, the chancellor announced at least £2bn of the extra £20bn earmarked for the NHS by 2023 would go on mental health.
A new four-week target for access to CAHMS is also going to be piloted soon.
A spokesman for NHS England said services were improving and in the coming years another 70,000 children will be able to access support.