Jails in England and Wales are unsafe and "full of drugs", the Chief Inspector of Prisons has said.
Peter Clarke warned the issues affecting prisons around the country risk creating a "major obstruction" to the Government's reform plans.
He described a "failure of leadership" seen in some establishments and criticised a fall in how many of his recommendations are implemented.
Ministers are attempting to address a safety crisis behind bars amid soaring levels of violence and self-harm.
Last year Justice Secretary Liz Truss unveiled a wide-ranging programme of reforms, including a recruitment drive to add 2,500 frontline officers and measures to crack down on drugs and mobile phones.
Asked what was wrong with prisons, Mr Clarke told MPs: "Basically they are unsafe, they are full of drugs. We have an ageing population.
"The physical environment is appalling and there are far too many people in our prisons who are suffering from mental health issues.
"In my judgment those five issues will create a major obstruction to the reform programme."
Appearing at the Commons Justice Committee, Mr Clarke said the issues were frequently referred to in his office's reports.
He spoke of his frustration that recommendations made on the back of inspections appear to "fall on deaf ears" in some instances.
There has been a decline in the number of recommendations overall that have been implemented over the last year, MPs heard.
Mr Clarke added that "most worryingly" this trend was being seen in the area of safety.
He said: "We are now in a position where more of our recommendations on the subject of safety are not being achieved than are being achieved."
Mr Clarke said it was "absolutely clear" that there is a "failure of leadership in some prisons, not all".
He identified "visible leadership" as a key element.
"As I walk around the wings of a prison, if I get a group of prisoners approach me and say, 'Are you the governor?' that tells me an awful lot about the visibility of the leadership in that prison," Mr Clarke said.
The scale of the problems in prisons was laid bare last week with the publication of official figures showing suicides, assaults and self-harm have all surged to new peak levels.