Drugs previously known as "legal highs" have been linked to the deaths of at least 79 prisoners, a watchdog has revealed.
New psychoactive substances (NPS), which mimic the effects of drugs such as cannabis, have been identified as a major factor behind the surge in violence and self-harm across jails in England and Wales.
The office of Prisons and Probation Ombudsman Nigel Newcomen has now identified 79 deaths which occurred between June 2013 and September 2016, where the deceased inmate was known or strongly suspected to have taken the drugs before their death, or where their NPS use was a "key issue" during their time in prison.
Of these investigations, 56 related to self-inflicted deaths.
The last update put the total number of prisoner deaths linked to NPS at 58 between June 2013 and January last year.
Mr Newcomen said: "Establishing direct causal links between NPS and the death is not easy, but my investigations identified a number of cases where my clinical reviewers considered that NPS led to psychotic episodes which resulted in self-harm.
"In other cases, NPS led to bullying and debt of the vulnerable, also resulting in self-harm."
He said the substances, which he has described as a "game-changer" for prison safety, are a "scourge" in jails.
"Reducing both their supply and demand for them is essential," Mr Newcomen said.
NPS were widely known as "legal highs" before laws criminalising the production, distribution, sale and supply of the drugs were introduced last year.
Prisons Minister Sam Gyimah said: "The current level of drugs in our prisons is absolutely unacceptable - causing untold mental and physical damage to prisoners, fuelling violence against staff, and jeopardising important reforms to transform prisons into places of reform and rehabilitation."
Measures introduced to tackle the problem include mandatory testing for psychoactive substances, the training of over 300 specialist drug dogs, and the introduction of new laws targeting those who smuggle drugs into prisons.