The rate of self-harm among children at Feltham Young Offenders Institution (YOI) has surged amid a deterioration of safety and care at the jail, the prisons watchdog has warned.

Inspectors said that despite improvements seen in 2018, there had been some “drift” during a subsequent period without a governor.

It found that levels of harm at Feltham A, the unit which holds boys aged 18 and under, had more than doubled, with 76 incidents over the previous six months compared to 34 recorded at the previous inspection.

More than one in four (26 per cent) children were locked in their cells during the working day, while only around a third could shower every day, inspectors said.

The report stated that care for those in need in the unit, which held 148 children at the time of the inspection, was “reasonably good”, but would have been better if “such children were not locked up, often alone, for extended periods”.

A survey carried out by the watchdog found 13 per cent of children said they felt unsafe, while nearly two-thirds said they had been physically restrained and many suggested they felt victimised by staff.

Levels of violence had increased significantly since 2018, with 230 incidents of violence in the six months to the inspection, a return to the high levels reported in 2017. Inspectors noted that not enough had been done to identify the reasons behind the increase in violence.

It comes as the Howard League for Penal Reform, which runs a free and confidential legal advice line for children and young people in custody, said it had received 57 calls in the last year either from or on behalf of children in Feltham – the highest call rate about any children’s prison.

In one case, a child reported only being allowed to have two showers a week and one change of clothing a week. In another case, a boy’s mother expressed concern that her son was spending more than 23 hours a day in his cell, not getting access to education and not receiving appropriate medicated shower gel to deal with a longstanding skin condition.

Based on the findings, Chief Inspector Peter Clarke said he was taking the “unusual step” of returning to the jail in July to carry out a survey followed by a full inspection, saying: “I have come to the conclusion that in all the circumstances it is a necessary and appropriate course of action”.

Mr Clarke added: “We understood the overriding need to keep children safe from one another, but such arrangements were having an impact on all aspects of the regime, limiting opportunities for children to make any progress.

“Oversight and scrutiny were lacking and we found evidence of poor practice, including the use of pain-inducing techniques, that had not been accounted for.”

Responding to the report, shadow justice minister Imran Hussain said: “There are deep concerns about the profound failure of the youth custodial estate to keep vulnerable children safe. The shocking findings in this report will only add to these fears.

“Labour has repeatedly warned about the youth estate and Feltham in particular, and the significant increase in violence and appalling levels of self-harm identified in this report demonstrates the dire crisis the youth custody estate is in."

Frances Crook, chief executive of the Howard League, said: “It is absolutely depressing, but not surprising to see Feltham once again slump into further problems around safety, after slight improvements registered by inspectors last year.

“It is a scandal that thousands of children have gone through its walls over the decades and experienced poor treatment, violence and abusive treatment.

“With such high levels of violence, escalating self-injury among children, and nearly two-thirds of boys experiencing physical restraint, the truth is Feltham is an irredeemably flawed institution.”

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Dr Jo Farrar, chief executive of HM Prison and Probation Service, said: “HMYOI Feltham A is a complex and challenging place, and we are pleased that inspectors have recognised the work of the new governor and her commitment to driving forward improvements at the prison.

“We are taking urgent action to address the concerns raised – this includes opening a specialist unit to provide interventions and support for the most challenging young people, and providing each offender with a dedicated officer to better help their rehabilitation.