The Mental Welfare Commission for Scotland said NHS Lanarkshire had the highest number of underage mental health admissions to non-specialist wards of any Scottish health board in 2011/12.
The figures refer to cases where under-18s with mental health problems, ranging from schizophrenia to eating disorders, are admitted on to either adult psychiatric wards or paediatric wards with no mental health service.
Such cases rose 10% in Lanarkshire between 2010/11 and 2011/12, in breach of the health board's legal duty to provide "age appropriate" services to children and adolescents suffering psychiatric problems.
There were 32 separate admissions of young people on to non-specialist wards during 2011/12, involving 27 individual patients
This means some were admitted on to unsuitable wards more than once due to a shortage of beds. It bucks a national downwards trend.
The commission said Lanarkshire's figures may have been influenced by the deaths of 15-year-old Neve Lafferty and Georgia Rowe, 14, after they absconded from the Good Shepherd Centre in Bishopton in October 2009. They had a pattern of self-harm and suicide attempts.
It is thought psychiatrists may have become more likely to admit youngsters into hospital for treatment following the case if they were displaying similar patterns of behaviour, but in doing so highlighted NHS Lanarkshire's con-tinued struggle to provide sufficient cover for the young.
The health board said it was working hard to reduce the number of admissions to adult mental health beds and added it closely monitored all admissions of young people.