Young offenders confined to prison cells for nearly 24 hours a day are vulnerable to deficiencies that cause Victorian-era bone deformities like rickets, inspectors fear.

The Care Quality Commission’s (CQC) outgoing chief inspector of general practice voiced concerns about inmates “locked up in their cells on their own for most of the day”, which were echoed by prison reform campaigners.

Professor Steve Field raised the issue in an interview with The Independent before stepping down from his role overseeing inspection of GP surgeries, dentists, sexual health, and services in the justice system.

He highlighted HMP Feltham, the west London juvenile prison and young offenders institute, as a site where inmates were at risk of vitamin D deficiency.

Sunlight on skin allows the body to generate vitamin D – a lack of which can cause rickets, a condition more common in the 19th century, which leaves victims with bent and malformed legs.

UK prison conditions: in pictures Show all 8 1 /8 UK prison conditions: in pictures UK prison conditions: in pictures A cell covered in graffiti at HMP Liverpool. PA UK prison conditions: in pictures HMP Liverpool has some of the worst conditions inspectors have seen. HM Inspectorate of Prisons/PA UK prison conditions: in pictures A broken window in a cell at HMP Liverpool HM Inspectorate of Prisons/PA Wire UK prison conditions: in pictures A shower unit with protruding electric cable at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures Litter at HMP Liverpool HM Inspectorate of Prisons/PA Wire UK prison conditions: in pictures A wall damaged by damp at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures A pool table at HMP Liverpool HM Inspectorate of Prisons/PA UK prison conditions: in pictures A sign for HMP Liverpool, where drones are seized at a rate of more than one a week. The prisons watchdog flagged up the impact of the remote-controlled flying devices at HMP Liverpool. PA

“What concerned me when I was in Feltham was that there were lots of young offenders there locked up for most of the day,” said Mr Field.

“Young kids, locked up in their cells on their own for most of the day. They weren’t getting the physical and mental stimulation they should do. They weren’t getting access to sunlight, so I was worried about vitamin D and all sorts of things.”

An inspection of Feltham by prison inspectors in 2017 found staff shortages and regime restrictions led to some inmates being locked in their cells for more than 22 hours day. Inspectors said a third of inmates in the prison’s adult wing, which holds men aged 18 to 21, were locked up for a core of the day and had “insufficient time in the open air”.

A report last year on Feltham’s young offenders’ institute found most boys were allowed out of their cells for seven hours on weekdays. But inspectors were “not confident that all boys had sufficient time in the open air”.

The Howard League for Penal Reform said it “had longstanding concerns that children and young adults in custody are being failed by prison regimes”.

Andrew Neilson, director of campaigns, told The Independent: “Young people being held in cells for long periods with nothing to do is far from unique to Feltham and can be seen in other prisons around the country. At the very least outdoors activity and other physical and mental stimulation should be essential elements offered within these institutions.

“More broadly however, we must question why these young people are in prison in the first place, particularly when they are children under the age of 18.”

Vitamin D deficiency is a growing problem in the UK, even for people with the liberty to go outside when they want. While rickets is primarily a risk in early childhood, deficiency can also cause problems during the puberty growth spurt.

Government advisors have said the country is not sunny enough for Britons to maintain health levels. They advised people – particularly those with darker skin, who are at greater risk of vitamin D deficiency – to take supplements.

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A lack of staff to supervise the release of inmates from cells to exercise and take part in other activities has been linked to a surge in disruptive behaviour, violence and self-harm in Britain’s prisons. The number of operational staff, from officers to governor grades, plunged 18 per cent – from 33,962 to 27,911 – between 2010 and last year. On average, prisons in England and Wales now witness an assault every 20 minutes, and a prisoner takes their own life once every four days.

While provision of healthcare is improving in prisons, Mr Field said there was “no doubt” it remained a problem. He stressed this was just as important for adults, including the increasing elderly population behind bars – an group MPs and penal reform campaigners have said the government is failing.

The Prison Service said it worked with the NHS and Department of Health to deliver care for offenders.

A spokesperson added: “Ensuring offenders have nutritious meals and sufficient opportunities for exercise is crucial.

“At Feltham, offenders have the chance to take part in football, rugby and personal training courses. In the last month the prison has launched a Saturday morning parkrun and a cricket pilot, with an indoor rowing club due to start within weeks.”