Children held at a prison run by the outsourcing giant G4S were subjected to “degrading treatment” and “racist comments” at the hands of staff who were under the influence of illegal drugs, a damning report by the education watchdog has revealed.

Ofsted inspectors who visited Rainsbrook Secure Training Centre in Northamptonshire earlier this year said staff had behaved “extremely inappropriately” towards the young people in their care, causing them “distress and humiliation”.

Frances Crook, the chief executive of the Howard League for Penal Reform, described the inspectors’ conclusions as “the worst report on a prison I have ever seen”. Six members of staff at the jail were sacked in the wake of the incidents.

The report said that doctors’ advice was often overruled by senior managers, meaning that one young inmate who suffered a fracture – potentially as a result of being restrained by guards – did not receive medical treatment for 15 hours.

The centre, which is managed by the private firm G4S, is designed to house up to 87 male and female young people aged between 12 and 18 who have been given a custodial sentence or are on remand.

Ofsted said the full details of a number of incidents – some of which involved staff in leadership roles – were so serious that they were being withheld to protect the children’s confidentiality. Its report gave Rainsbrook the lowest possible rating of “inadequate”.

G4S said the incidents had taken place well before inspectors visited and that a new management team had since been put in place. Rainsbrook is currently being investigated by the Prisons and Probation Ombudsman.

Ms Crook said: “This is the worst report on a prison I have ever seen because it is a catalogue of abusive practices that have been inflicted on young children who have no escape.

“I visited Rainsbrook some years ago and found it to be claustrophobic and obsessed with security, a recipe for exactly the disaster now happening. These child jails run for profit are secretive and should never have been set up in the first place. Rainsbrook should be closed immediately. No child is safe in this jail.”

Frances Crook, chief executive of the Howard League for Penal Reform, said: “This is the worst report on a prison I have ever seen because it is a catalogue of abusive practices that have been inflicted on young children who have no escape. (Getty)

The Ofsted report added that after some of the incidents there had been “unacceptable and inexplicable” delays in removing staff from their roles caring for young people. In some cases the outcome of internal investigations had been “too lenient”, inspectors said.

In the space of six months, children held at the centre were restrained 166 times, the report said, with 72 of these in response to an inmate self-harming. A stash of contraband DVDs discovered at Rainsbrook was likely to be the result of smuggling by staff, the report added, raised concerns that some may have “colluded” with young inmates to “elicit [their] compliance by wholly inappropriate means”.

The Ministry of Justice said the report raised “issues of serious concern”. A spokesperson added: “The safety and welfare of young people in custody is vital. Urgent action will be taken to tackle the unacceptable failures raised in this report.”

Lin Hinnigan, chief executive of the Youth Justice Board (YJB), added: “Earlier this year, Ofsted informed the YJB of serious concerns in performance at Rainsbrook STC. As the safety and wellbeing of young people in custody is of paramount importance, and the YJB sets high standards to ensure it is maintained, we immediately required G4S to address the issues swiftly and effectively.

“Rainsbrook has new leadership in place and an action plan to improve recruitment and training is being implemented. We are confident that Rainsbrook will return to the high levels of performance and care it previously delivered.”

G4S’s director of children’s services, Paul Cook, said the report was “extremely disappointing report for everyone connected with Rainsbrook”. He added that many of the issues raised by Ofsted had already been dealt with by the time that its inspectors visited, so the watchdog’s report should be considered more of a “retrospective review”.