The girl, who had a criminal record for burglary and theft, was sent to an open ward as an informal patient by Kirklees council, West Yorkshire, for assessment after social workers said they feared she was suicidal.
Lawyers acting for the girl, now aged 16, argued that she had been wrongly held for 18 days in an acute adult psychiatric ward at St Luke's hospital, Huddersfield, in 1989. Elizabeth Lawson QC, for the girl - referred to as 'C' - said that the 1983 Mental Health Act permitted informal treatment of a person with a mental disorder, but not informal admission for assessment.
But Lord Justice Lloyd ruled the council had properly exercised its powers to act in the best interests of a child in its care. Lords Justices Stuart-Smith and Far quharson agreed.