Huge quantities of drugs and mobile phones filtered into a prison because of a "serious compromise" in security, an independent report has revealed.
The security breach was uncovered at the mobile temporary unit at Camp Hill on the Isle of Wight last year but has since been dealt with.
The rise in the amount of drugs and mobile phones at the Category C site had an impact on the regime and the level of violence, the prison's Independent Monitoring Board (IMB) report said.
Drug debts and substance abuse leads to intimidation, bullying and violence, and roof-top protests were sparked by inmates scared of violence and wanting to be moved, it added.
The board said it was concerned about the number of roof-top incidents at Camp Hill, resulting in specialist teams from the mainland being deployed at high cost.
More than 100 vulnerable spots where prisoners could gain access to out-of-bounds areas were identified in a recent survey at Camp Hill. But this was not deemed unusual as it is a Category C level jail, the board said.
HMP Isle of Wight was formed following the merging of the former Parkhurst, Albany and Camp Hill prisons in April 2009 in an effort to cut overall budget costs.
The report said: "The amount of drugs freely available, especially in Camp Hill site, is wholly unacceptable, and causes bullying and prisoners living in real fear of their safety.
"Parkhurst and Albany sites suffer from illegal drugs less, but the sale of prescribed drugs, by one prisoner to another, causes great problems and funds should be provided in order to properly control the issue and taking of prescribed medication."
The board said it found it "totally unacceptable and unbelievable" that, in an establishment housing almost 1,700 prisoners, there was only one dog handler.
"Taking normal working hours, annual leave and training into account, this leaves the jail unprotected for a cumulative total of over 130 days a year," the report went on.
It urged ministers to review the situation urgently.
The IMB praised prison and police intelligence for foiling an elaborate escape plan using a helicopter last summer.
Murderer Brian Lawrence, who was convicted of killing a friend of his ex-lover and hiring a hitman to kill two others, used letters, Sudoku puzzles and writing in lemon juice as invisible ink to disguise his plans.
The operation was thought to involve using a nearby pop concert as diversion tactics because musicians regularly arrived and left by helicopter.
A Prison Service spokesman said: "We thank the Independent Monitoring Board at HMP Isle of Wight for their report, which will be fully considered by ministers. We will respond to the board in due course."