But the judges who made the judgment on Tuesday said yesterday that they considered it "difficult to find any argument" for allowing young offenders to mix with adult criminals.
About 80 young women are held in adult prisons instead of young offender institutions because there are only eight women jails with such units, creating travelling difficulties.
But the ruling is also believed to apply to the much larger population of 1,500 young male offenders also held in adult jails. Complying will create a logistical nightmare for the already over-stressed prison system.
The Prison Service was yesterday arranging to designate more prison accommodation as young offender institutions, but said that would inevitably increase the pressure on the adult system. The details of the judgment were released yesterday. It enforces a section of the 1982 Criminal Justice Act which states that offenders under-21 who are sentenced to detention in a young offender institution must be sent to one.
There is a power to detain in prison in individual cases, but Mr Justice Sedley and Mr Justice Anstill held that the Home Office was acting unlawfully in using this power as a blanket policy covering all young women. In practice, young women have been automatically sent to adult jails pending removal to young offender units.
Leave to appeal was granted after Robin Tam, for the Home Office, said the case was "of considerable importance."
The young woman at the centre of the case, a 16-year-old known as F, spent 15 days in Risley jail, Cheshire, but has now been found a place at Styal women's prison in Cheshire which has a young offender unit.Reuse content