An increasing number of under-18s with mental health problems in England are being treated on adult psychiatric wards, according to new statistics.
Treating young people in such units should happen only in exceptional circumstances and the Department of Health had promised this would stop by 2010.
However, through Freedom of Information requests, the BBC and online journal Community Care found the number of under-18s being treated in adult units was in its hundreds - and rising.
Data returned by 51 of the 58 NHS mental health trusts in England showed that 350 under-18s have been admitted so far to adult mental health wards in 2013-14, compared with 242 two years earlier.
The DH insist that children and young people's mental health remains "a priority". A spokesman added: "Our mental health crisis care concordat reinforces the duty on the NHS to make sure that people under 18 are treated in an environment suitable for their age, according to their needs."
Yet, NHS England national clinical director Dr Jacqueline Cornish says that changes cannot come soon enough and that treating children with mental health problems in adult settings was "totally unacceptable in the majority of cases". She also pledged to launch a three-month "rapid review" into the situation.
The figures also revealed that 12 under-16s have been admitted so far in 2013-14, compared with just three in 2011-12.
Of 18 trusts that provided out-of-area placement data, 10 had sent children more than 150 miles away for care.
Sarah Brennan, chief executive of the charity Young Minds, said the increase in children placed on adult wards was predictable following cuts to early intervention services over the last four years.
"The lack of help early on means we are letting children's problems escalate to serious levels," she said.
"This, alongside the lack of accurate data about the mental health needs of our nation's children and young people, means commissioning has been based on out-of-date, inaccurate information, leading to out-of-date provision."