Four in 10 mental health services for children are failing, with youngsters facing long delays for treatment, according to a Government-ordered review.
The study, from the Care Quality Commission (CQC), found many services set their own targets for how quickly youngsters should be seen, leading to children's mental health deteriorating while they were forced to wait.
Some children endured 18-month waits for help, while in one service there was a wait of over a year (493 days) for cognitive behavioural therapy and a 610-day wait for family therapy, the report said.
The analysis showed that "some children and young people are waiting an extremely long time to access the specialist care and support they need", while specialist community services have "set their own waiting time targets" and "there is significant variation in these targets".
One child may be seen within 35 days in one service, but a child in another part of the country may wait 18 weeks or more, it said.
The report also heavily criticised arrangements for crisis care for those in most desperate need.
Sometimes, this care was only available during working hours of 9am to 5pm, leaving night-time care provided by adult psychiatrists who lacked expertise in children's mental health.
The study added: "In inpatient services, our inspectors found that children and young people could not always be placed in the setting that would best meet their needs.
"The demand for inpatient beds outstrips availability in some parts of the country where fewer beds are available.
"As a result, some children and young people are being admitted to adult wards as there are no beds available in wards for people their age."
The report, which was partly based on 101 CQC reports of specialist child and adolescent mental health services (CAMHS), rated 39% of services (26 in total) as requires improvement and 2% (one service) as inadequate when it came to responsiveness - whether care and treatment is given in a timely way.
It said Public Health England estimated that only a quarter of children and young people who needed treatment for a mental health problem were able to access it.
It also told how, sometimes, children and young people "are repeatedly referred to different parts of the system after several services tell them they fail to meet the threshold for support".
The report highlighted key areas of concern in children's mental health, including rising levels of self-harm, the risk of suicide and how the internet and social media may be contributing to poor mental health.
The CQC did find that when young people were able to access specialist services, they often received good quality care.
But there were still significant concerns over safety, such as dirty services putting people at risk of infection, or environments that put children at risk of harm.
Claire Murdoch, mental health director for NHS England, said: "Around the country, these critical services are beginning to expand and improve, with three quarters of young people now getting urgent eating disorder care within one week.
"But NHS England has also been explicit about the scale of unmet need, which recent improvements have inevitably only been able to begin to tackle.
"It's going to take years of concerted practical effort to solve these service gaps - even with new money - given the time it inescapably takes to train the extra child psychiatrists, therapists and nurses required."