The decision is likely to cause the Prison Service significant disruption and expense and force officials to find new places for female offenders aged under 21. The ruling is expected to affect up to 80 young female inmates who are at present in adult women jails.
Eight of the 14 women's prisons have specialist wings for young offenders, but they are full as the prison population continues to accelerate. There are now 2,799 female prisoners in England and Wales, of which 382 are aged under 21.
Mr Justice Sedley and Mr Justice Astill yesterday ruled that the Home Office policy under which all young female offenders are sent automatically to one of five adult prisons for assessment and allocation to young offender units was unlawful. Young male criminals are usually sent to specialist institutions rather than adult jails.
The Home Office will now be forced to reconsider the policy.
The move follows the case of a 16-year-old who spent 15 days in Risley Prison, Cheshire, sharing cells with adult prisoners, before being released on bail.
The judges were told that the girl, referred to as F, had now been found a place at Styal women's prison in Cheshire, which had facilities for juveniles, and was no longer threatened with return to Risley.
The girl was sentenced to eight months for robbery, assaulting a police officer, disorderly behaviour and other offences.
In a judicial review hearing, her counsel, Ian Wise, argued that the use of adult women jails for young offenders should be exercised only in individual cases and in exceptional circumstances, whereas the practice was applied to all juvenile girls.
Mr Wise said: "There are, of course, no young offender institutions specifically for females, only female prisons which have been designated as having a dual purpose for adult and young offenders."
The judges said they would give reasons for their decision to rule the practice unlawful at a later date, but added that in the meantime, the Home Office "may be able to consider what it can and should do to meet the legal situation as we hold it to be".
Robin Tam, for the Home Office, said the fact that all girls were sent initially to prison did not mean their cases were not being individually considered. The prison authorities needed to assess each case to make a rational decision on placement.
The case follows a recent report by Sir David Ramsbotham, the Chief Inspector of Prisons, in which he expressed concern about the mixing of girl offenders with adult female prisoners.
The Howard League for Penal Reform, which supported the case, said the practice is in breach of the UN Convention on the Rights of the Child.Reuse content