Prison staff will not enter a cell alone at night even to save an inmate’s life, an inspection has revealed.

Cookham Wood Young Offenders’ Institution, which houses teenage boys, has been criticised for its “lacklustre” approach to preventing suicide and self harm in a report.

Inspectors at the jail in Borstal were "surprised" to find a lack of attention in key areas, particularly after the death of a 15-year-old inmate last year.

An unannounced inspection at the prison, in Sir Evelyn Road, found that supervision for those deemed at risk was often carried out by agency staff who did not carry keys and could not enter a cell immediately.

Some night staff said they would not enter a cell alone when on patrol, even to save a life. Nurses were found to have "limited knowledge" of the boys they were observing and there was minimal engagement with prisoners.

Nick Hardwick, chief inspector of prisons, said that while real progress had been made in other areas, the measures for suicide prevention were "an exception to this positive picture".

He added: "Despite the tragic death of a young person in 2012, some arrangements were surprisingly lacklustre and needed improvement."

Alex Kelly, who had previously been identified as a suicide risk, died after being found unconscious in his cell in January last year.

A postmortem gave the cause of death as irreversible cerebral hypoxia (lack of oxygen to the brain) caused by suspension. An inquest into his death was opened in February last year but is currently adjourned.

Inspectors also expressed concern that some areas highlighted by a report into his death by the Prison and Probation Services had not been addressed.

The prison was, however, said to have good structures to promote safety, positive relationships between inmates and staff and a "broad and relevant" curriculum.

“This is a good report about an institution that has made real progress despite needing to manage challenging young people in a poor environment” - prison inspector

There was also praise for improvements in resettlement and the use of creative and original ideas.

Mr Hardwick added: "This is a good report about an institution that has made real progress despite needing to manage challenging young people in a poor environment."

The report also expressed concerns at high levels of force and violence, and the length of time some teenagers spent in separation.

The former women’s prison, which once housed Moors murderer Myra Hindley, was opened in 1978 and became a young offenders’ institution for male offenders aged 15 to 18 in 2008.

A Ministry of Justice spokesman said: "As the chief inspector acknowledges, the level of self-harm at Cookham Wood is low.

"However, we are not complacent and are working hard to implement the recommendations in the inspection report."