The High Court is to hear a challenge on Tuesday 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 judicial review action has been brought on behalf of the child, who cannot be named for legal reasons, by the Howard League for Penal Reform.
The league is "working for less crime, safer communities and fewer people in prison".
It said in a statement: "For long periods of his time in Feltham Prison, the boy ... has been locked alone in his cell for 23-and-a-half hours a day. He was denied any educational provision for months."
The charity says the Government has "conceded that, for long spells of the boy's time in custody, aspects of his treatment were unlawful - but it is still fighting the claim".
The statement added: "The Equality and Human Rights Commission has been granted permission to intervene in the case, which raises serious issues about keeping children in prolonged solitary confinement not previously considered by courts in the UK."
It will be argued at a hearing before Mr Justice Ouseley in London that the boy's treatment is in breach of the United Nations' Mandela Rules, which prohibit the use of solitary confinement for children.
Howard League chief executive Frances Crook said: "This is a widespread problem and it is getting worse.
"In just the last week, several more children, held in prisons across the country, have asked the Howard League for help because they are in almost total isolation."
He said: "Some of the children we have represented have been in solitary confinement for up to nine months and have been subject to very similar regimes to the boy in this case.
"They have been allowed out of their cells for about 30 minutes a day, with little or no access to education, limited access to exercise or fresh air, and no association or meaningful intervention."
The Howard League says the UK is "out of step with a growing international consensus that children should never be placed in solitary confinement".