Britain's prison watchdog warns of inmates on drugs and living in 'squalid' conditions

Mr Clarke, Chief Inspector of Prisons in England and Wales, highlighted the prevalence of drugs behind bars, saying the "seeming inability" to keep them out has been a major factor in declining safety standards.

Too many prisons still lack an adequate strategy to tackle drug supply, his assessment said.

Inspectors had seen many inmates who were "obviously" under the influence of drugs during visits.

Mr Clarke's annual report for 2016/17 said debt, bullying and self-segregation by prisoners are commonplace.

"This has all been compounded by staffing levels in many jails that are simply too low to keep order and run a decent regime that allows prisoners to be let out of their cells to get to training and education and have access to basic facilities," he said.

"When a person is sent to prison, the state accepts responsibility for their well-being, including their physical and mental health, safety and education. There is clear evidence that for too many prisoners, the state is failing in its duty."

Staffing levels have repeatedly been highlighted as violence and self-harm surged to record levels across the estate.

Assaults on staff jumped by 38% last year to 6,844 incidents.

At the height of the crisis last year, there were a flurry of disturbances.

Of the 29 local and training prisons inspected during the year, inspectors judged 21 of them to be poor or "not sufficiently good" in the area of safety.

Mr Clarke criticised "squalid, dirty and disgraceful" conditions in some establishments.

Far too often, men are sharing a cell in which they are locked up for up to 23 hours a day, his report said, adding that on several occasions, prisoners pointed out insect and vermin infestations.

Mr Clarke said that in many cases, the response to his inspectorate's previous recommendations has been "unforgivably poor".

The number of recommendations that had been fully achieved was lower than the number not met for the first time in 2016/17.

Mr Clarke said there is a "huge gap" between accepting his recommendations and achieving them.

This "leads to the inevitable suspicion that there is a degree of lip service being paid to our recommendations", he added.

Last year, ministers launched a recruitment drive to add 2,500 frontline officers as part of a wide-ranging package of prison reform measures.

At the end of March, there were 18,403 "full-time equivalent" staff in frontline categories, which was an increase of 75 on the last year and 515 on the previous quarter.

The chief inspector said additional officers "can only be of help" but he stressed that staffing numbers are "not the only issue".

His report also raised the alarm over the state of the youth custody estate, saying the speed of decline at secure training centres and young offender institutions had been "staggering".

Youth Justice Board statistics show rising levels of self-harm and assault rates, and Mr Clarke disclosed that earlier this year, he had felt compelled to raise his serious concerns about findings in the youth estate with ministers.

"By February 2017, we concluded that there was not a single establishment that we inspected in England and Wales in which it was safe to hold children and young people," he said.

Mr Clarke gave a more positive assessment of women's prisons, saying they showed strong outcomes for safety and resettlement. Open prisons and high-security establishments also "inspect well".

Elsewhere, the report warned that new psychoactive substances, previously known as "legal highs", are beginning to have an impact in immigration detention.