The High Court is to hear an urgent challenge over a teenage boy's "prolonged solitary confinement" in prison.
A penal reform charity says the boy has been locked in his cell for more than 23 hours a day at Feltham Prison in west London.
The Howard League for Penal Reform announced in a statement that the High Court has agreed to hear its judicial review action brought on behalf of the child, who cannot be identified for legal reasons.
According to the charity, the boy has been "permitted no contact with any other child" during the "short periods out of his cell".
The Howard League's legal team will argue at a hearing in London that the boy's "removal from association and the lack of educational provision is unlawful".
It said the High Court had granted permission for the case to be heard urgently but no date has yet been set.
Mr Justice Ouseley considered the judicial review application and ordered an expedited hearing.
When the action reaches court it will be argued that the treatment of the boy is in breach of the United Nations' Mandela Rules, which prohibit the use of solitary confinement for children.
The charity says solitary confinement of children in England and Wales is "out of step with other countries and there is widespread recognition that change is needed".
A "statement of grounds" prepared for the case says that the "practice of informally removing children from association without any statutory basis, and of subjecting children to solitary confinement, appears to be common at Feltham as well as other child prisons".
The statement continues: "If the claimant is correct that such a practice is unlawful, and indeed if it constitutes cruel, inhuman and degrading treatment of children, it is a matter of real importance that it be remedied by the courts."
The Howard League, a national charity "working for less crime, safer communities and fewer people in prison", says that in the past two years its lawyers have supported at least six teenage boys under the age of 18 who have been in solitary confinement for periods ranging from weeks to more than six months.
Chief executive Frances Crook commented: "Caging children for over 22 hours a day is unacceptable.
"All the evidence shows that it can cause irreparable damage. This practice must cease."