Prison inspectors exposed yesterday the shameful effects of government cuts after a London jail was found to be understaffed, overcrowded and infested with vermin.

HM Inspectorate of Prisons inspectors found that cuts at HMP Pentonville in north London was having "serious consequences" on staff and inmate safety, with high levels of drug use among prisoners and wardens losing track of prisoners' whereabouts.

The inspectors said that around 1,236 men were held in the prison at the time of the unannounced inspection last summer - 35 per cent above its normal accommodation level.

Prison workers' union POA welcomed the report and said it vindicated its claims that the Ministry of Justice's (MoJ) cost cutting was a recipe for disaster.

General secretary Steve Gillan said: "Budget cuts create overcrowding which leads to increased tensions and assaults on staff and inmates, increased levels of self-harm and deaths in custody. It means we are not rehabilitating inmates but warehousing them.

"The government cannot say it wants a rehabilitation revolution then continue to slash the budgets."

The inspectors found that the prison was operating at well below its agreed staffing levels as sickness and absence were high among workers.

It said that "the staffing reductions the prison were having a number of serious consequences." It pointed out that on average 19 prisoners self-harmed each month and around 60 prisoners were on suicide and self-harm watch at any one time.

In addition, the prison had "significant, easily visible vermin infestations" with many men sharing "small, dirty, badly ventilated single cells" with broken furniture and broken windows.

Chief inspector of prisons Nick Hardwick said: "At the time of the inspection the prison was going through a particularly difficult time as it made the transition to new staffing levels.

"Nevertheless, it is clear that Pentonville cannot operate as a modern 21st-century prison without investment in its physical condition, adequate staffing levels to manage its complex population and effective support from the centre."

The National Offender Management Service, the executive agency of the MoJ, which runs prisons and probation services in England and Wales, said that the prison had since been going through a transitioning period and "Pentonville will receive the support it requires to build on the progress made and to address the further recommendations set out in this report."

But Howard League for Penal Reform chief executive Frances Crook said it was a sharp reminder that the MoJ "was pushing through unsustainable budget cuts without addressing the overall demand that prisons like Pentonville face in the first place.

"That such a failing institution lies in the heart of 21st-century London should shame us all."