Newly freed prisoners are turning back to a life of crime to repay drug debts built up in prison – with some having smoked up to £800 a day of illegal substances like "zombie drug" spice.

A damning inspection report reveals how bored inmates are consuming between five and eight grams of new pyschoactive substances (NPS) every day at a price of up to £100 a gram.

The now-banned "legal highs" have flooded prisons in England and Wales and have been blamed for contributing to record levels of violence and self-harm in jails.

They are often cheaper and easier to smuggle into prisons than drugs like cannabis and have become lucrative contraband for gangs running prison black markets.

A report by HM Inspectorate of Probation, published on Wednesday (29 November), said: "Prisoners may use NPSs to relieve boredom or may be bullied into taking the drugs by others.

"Regular users develop a tolerance, which can result in their habits escalating and increased debt to dealers. This can lead to self-harm, violence and instability in prisons and approved premises."

It added: "[Inmates'] level of dependency led to debts building, with prison prices being up to one hundred pounds per gram.

"Those in debt were often involved in violent incidents where threats were made to family members, with some stating that they offended to repay the debts they had built up in prison."

Previous inspection reports have revealed how some inmates are refusing to leave their cells because drug debts have left them vulnerable to bullying and violence.

Dame Glenys Stacey, chief inspector of probation, said the problems behind bars continued upon release into the community.

She said: "We found that probation staff and even some substance misuse service staff had a low level of awareness of NPS.

"Screening tools for identifying drug use were not geared to NPS. Probation staff did not have structured, in-depth training about NPS and how to deal with dependency, and lacked the confidence and knowledge to quantify the problem and to address it."

Probation officers reported spice addicts turning up for interviews "unable to engage in conversation, shaking, sweating and at times behaving in a violent and aggressive manner".

One probation officer said: "People are crazy when they are under the influence. One confused me for a fire hose."

The HM Inspectorate of Probation report also said a lack of training meant probation officers were not assessing the risk to children and others from offenders addicted to spice.

It comes after the Chief Inspector of Prisons Peter Clarke noted a spike in violence and self-harm in prisons across England and Wales in his 2015/2016 annual report, adding: "It is clear that a large part of this violence is linked to the harm caused by new psychoactive substances which are having a dramatic and destabilising effect in many of our prisons."

First sold as a legal high, spice is now banned by the government but can still be bought online.

The drug, along with other new psychoactive substances, has been linked to 79 deaths of inmates between June 2013 and September 2016 and is blamed for causing numerous cases of psychosis.

Even prison guards have complained of falling ill after accidentally inhaling the fumes.

Despite successive governments vowing to clamp down on illegal substances behind bars, inmates continue to say they "flood" jails – with one former prison governor describing spice as "the biggest health problem facing prisons".

A Ministry of Justice spokesman said: "We are working with agencies across the criminal justice system to address this problem."