Treating children in adult psychiatric wards is a "national scandal", says the Children's Commissioner for England.
By Alison Holt
Social Affairs correspondent, BBC News
Professor Sir Al Aynsley-Green told the BBC he fears children leave in a poorer condition than when they went in.
A report to be published next week by the charity Young Minds warns there are not enough emergency beds for children with mental health problems.
Almost 1,000 under-18s spend time on an adult ward in a year, with more than half of those admissions inappropriate.
Young Minds' report, prepared for the Children's Commissioner, details the experiences of sixteen young people, aged between 13 and 19, who have been treated on adult wards for their mental health problems.
Some were held in police cells or accident and emergency units whilst a search was made for a place which could take them.
Most felt frightened and confused with little information about what was happening to them.
The report also says placing vulnerable teenagers on an adult ward raises child protection issues, with some of the teenagers being offered drugs or facing sexual advances.
Sir Al said the situation is a "national scandal".
"It wouldn't surprise me if children leave adult wards worse than when they went in.
"Putting children in an age-appropriate and developmentally-appropriate environment can only be better for them."
'It was scary'
Jay Taylor spent three weeks on an adult psychiatric ward in March this year because doctors couldn't find her a bed at a young people's unit.
'Age-appropriate care is needed'
She said: "I was really scared at first, it was nothing like I had ever seen before."
Jay saw one person being held down by nurses and forced to take their medication.
She says there was a lot of shouting and aggression with some patients clearly very disturbed.
"It did get scary when they came towards you and were shouting and stuff, with being so young I didn't know what to do."
Jay was suffering from depression and anorexia and had virtually stopped eating and drinking, needing to be fed through a tube to regain her strength.
She was admitted to the adult psychiatric unit in Newcastle.
"I was vulnerable. I became more depressed, I became isolated within myself, I wasn't getting the help I needed."
In recent years, extra money has gone into child and adolescent mental health services but they are still under pressure, and finding a bed in an emergency is difficult.
Dr Barry Chipchase of Northumberland, Tyne and Wear NHS Trust, one of the child psychiatrists involved in Jay's case, said they do their best to find more appropriate places for under 18's - but sometimes they have no choice.
"We know that most adolescent units are almost always full all of the time.
"I know this because if I have a young person who needs admitting and we don't have a bed in this unit, I have to ring round a number of other units around the country who tell me the same thing - that they don't have a bed."