A prison is simply “warehousing” inmates after being left with inadequate staffing numbers, a report has warned.

The Independent Monitoring Board (IMB) at HMP Chelmsford found prisoners were spending hours in their cells instead of attending education and work programmes.

The board’s assessment said: “This engenders frustration and violence.

“With too few officers, all the prison can do is ‘warehouse’ prisoners rather than pointing them towards positive behaviour change.”

It also raised concerns about drugs and violence at the prison.

The board flagged up figures showing the number of assaults, including fights, went up from 162 in 2012 to 414 last year, while assaults on staff showed a 400 per cent rise.

The report said: “It is our view that the widespread availability of drugs, despite the best efforts of the prison to detect them and to prevent their use, and of bullying, often as a result of prisoner indebtedness, are primary causes of the reported violence, prisoner injuries and self-harm.”

In the past year, three prisoners have killed themselves and an inquest into the death of Dean Saunders which took place earlier this year found "serious failings" in the prison system led to his suicide.

The Prison Service said HMP Chelmsford has already taken significant action to address the concerns raised by the IMB.

A spokeswoman said: “A dedicated recruitment campaign is under way, and the prison has achieved their benchmark target of 172 prison officers in post.

“There has also been a significant investment in improving safety and reducing violence, including the introduction of body worn cameras, upgrading CCTV and installing additional drug netting in vulnerable areas.”