Chief inspector of prisons Nick Hardwick said the report into Wormwood Scrubs prison was "very disappointing".
He said the prison had found it hard to replace experienced staff who left during a major restructuring last year
Although he said there was "some recent evidence" of action being taken to improve performance, there remains "much to be done".
He emphasised the heavy demands being made on staff, saying: "Many staff appeared extremely stretched and some were clearly frustrated that they could not do more; others appeared to have lost focus on prisoners' needs."
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Kevin Davis, 53, ran a medical company and helped to treat a 15-year-old who had taken a drugs overdose despite not having qualifications.
CCTV shows the 19-year-old victim being struck by a car and thrown metres into the air, before he lands on the pavement and slides along it.