Leading judges are being asked to rule that a teenager's "prolonged solitary confinement" in detention amounted to "inhuman and degrading" treatment.

Lord Chief Justice Lord Burnett, Lord Justice Moylan and Lord Justice Singh heard today (Wednesday, November 7) that the boy was locked alone in his cell at Feltham Young Offender Institution in west London for "over 23 hours a day".

His counsel, Dan Squires QC, told the Court of Appeal judges that "for at least his first 55 days at Feltham" the boy - who cannot be named for legal reasons and is referred to as AB - was kept in "solitary confinement".

The appeal proceedings in London follow a High Court ruling in the teenager's case against the Justice Secretary in July last year.

Mr Justice Ouseley rejected a claim that his treatment amounted to a breach of Article 3 of the European Convention on Human Rights (ECHR) which states that no-one should be subjected to torture or to inhuman or degrading treatment or punishment.

He ruled that "there has at all times been a considered and proper justification for the removal from association", initially to "protect officers, then to protect officers from AB and AB from inmates whose anger he aroused by shouting abuse".

The judicial review action was brought on behalf of the boy, who is described as having "significant" mental health problems, by the Howard League for Penal Reform.

Mr Squires told the appeal judges that AB, now aged 17, issued proceedings in February last year when he was 15 and had been detained at Feltham since December 2016.

He said: "For at least his first 55 days at Feltham AB was held in 'solitary confinement'.

"He was locked alone in his cell for over 23 hours a day. He received no education and had access to no gym, psychological intervention or any purposeful activity.

"He was permitted no contact with other children."

Mr Squires said the widely accepted definition of prolonged solitary confinement was "confinement for more than 22 hours a day alone in a cell with meaningful social contacts and available stimuli reduced to a minimum".

The "clear consensus of medical and expert penological opinion is that subjecting a child to prolonged solitary confinement creates a significant risk of long-term and potentially irreparable psychiatric and developmental harm and is never acceptable".

That was "particularly so for a child with mental health problems such as AB".

The teenager had argued at the High Court that his solitary confinement was incapable of being justified and breached Article 3.

Mr Squires said Mr Justice Ouseley made "errors of law" in rejecting that argument, and his approach to international law and "medical/penological evidence" was "erroneous".

The QC told the three judges: "Had those errors not been made the learned judge would have been bound to hold that at least the first 55 days of AB's detention breached Article 3."

The teenager had also claimed that his solitary confinement breached Article 8 of the ECHR - the right to private and family life.

Mr Justice Ouseley ruled that young offender institution rules on both "removal from association" - which relates to being kept in a cell and not allowed to mix with others - and education were breached.

He announced: "I shall make a declaration that Article 8 was breached because the interference with his rights was not in accordance with the law."

Mr Squires said the judge had found there was a "purely procedural" breach of Article 8.

He said Mr Justice Ouseley had considered only whether the teenager's removal from association and not his solitary confinement was justified.

Appealing against that aspect of the High Court decision, Mr Squires submitted that if the judge had "considered the matter", he would have been "bound to conclude" that AB's solitary confinement "did not pursue a legitimate aim and was not necessary and proportionate" pursuant to Article 8.