By Chris Hogg BBC Health Correspondent

As many as a quarter of teenagers with mental health problems are receiving NHS care which their own doctors say is inappropriate, says government-funded research seen by the BBC.

Leading psychiatrists say the number of beds in child and adolescent mental health services for those who are most ill needs to increase by at least a third.

The warning comes as a United Nations conference in New York describes the growing problem of mental illness among teenagers as a "ticking timebomb".

Experts say that, unless addressed now, it will put extreme pressure on health services in future years.

The BBC spoke to teenagers who are say they are not getting the right treatment from the NHS.

Adult ward

One, "Mark", has suffered from depression since the age of 14, and was sent for treatment at psychiatric hospital near his home in Birmingham.

I felt betrayed in a way by society, and I was very suicidal

"Mark"

However, this was an adult psychiatric hospital, and he said he felt isolated by the presence of so many older people.

He said: "They were all in their 40s and 50s - it made me feel left out.

"There should have been a better place for my age group instead of being in a massive hospital full of older people."

He said he suffered a relapse of his mental problems because he felt he was not being helped.

"I felt betrayed in a way by society, and I was very suicidal."

Mother's pain

His mother, "Sue", told the BBC she was left to pick up the pieces.

"I was left there to look after him, to support him, to console him, day in and day out.

"It wasn't a therapeutic environment."

During the early 90s, a fifth of beds allocated to children and teenagers disappeared, and there are wide regional variations in those that remain.

It is quite inappropriate

Professor Peter Hill, Great Ormond Street Hospital

Psychiatrists say they often struggle to find young people a suitable bed.

On one in four occasions, the only available bed is on an adult ward, or even an ordinary paediatric ward.

Professor Peter Hill, a professor of child and adolescent psychiatry at Great Ormond Street Hospital in London, said that many patients were not receiving care in the right environment.

He said: "I have seen teenagers of 16 and 17 who are over six feet tall who are confused and frightened by their mental illness, on the same ward as you have two and three-year-olds on drips for the treatment of their leukaemia.

"It is quite inappropriate and the nurses on those wards are not necessarily, indeed not usually trained to nurse and manage or treat or respond to children and teenagers with mental illness."