A legal challenge over a boy locked up for 23 hours a day in a London prison has been given the go-ahead.

The high court has agreed to hear a judicial review brought by the Howard League for Penal Reform on behalf of the boy, who is being held in prolonged solitary confinement in Feltham young offenders prison in west London.

A judge has agreed that the challenge should be heard urgently after the penal reform charity successfully applied for a judicial review of the case involving the boy, who court documents only identify as AB.

A statement of grounds prepared for the court argues that the removal of the boy from association and the lack of educational provision is unlawful.

The grounds say that the practice of informally removing children from association with other prisoners and keeping them in solitary confinement is without any statutory basis. It adds that the practice appears to be common at Feltham as well as other juvenile prisons.

“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 statement continues.

Howard League lawyers have supported at least six teenage boys under the age of 18 who have been in conditions of solitary confinement for periods ranging from weeks to more than six months. This is believed to be the first judicial review of the practice.

Frances Crook, the chief executive of the Howard League for Penal Reform, said: “Caging children for over 22 hours a day is unacceptable. All the evidence shows that it can cause irreparable damage. This practice must cease.”

The supreme court ruled in 2015 that there were “well known” risks involved with solitary confinement and that prolonged solitary confinement – defined as lasting for longer than 15 days – was particularly harmful.

A chief inspector of prisons report in 2015 of Feltham found that a quarter of the imprisoned children were being managed on units under a restricted regime that meant they were unlocked for less than an hour a day. The inspectors described it as “in effect, solitary confinement on their residential units”.

The children’s commissioner and another watchdog, the National Preventive Mechanism, have criticised the widespread use of solitary confinement for children in prisons in England and Wales. They found that one third of imprisoned children spent time in isolation and criticised the “worrying number of instances where isolation was not subject to formal governance”.