The threat of violence has never been far away from crowded jails but the problem is now endemic. The rehabilitation of offenders cannot happen within an unstable system. As the former prison governor Ian Acheson says: “Broken staff cannot help fix broken people.” A key factor in the crisis of authority in UK jails has been the rise of Islamist gangs after the jailing of radicals for terrorist attacks and plots. Almost 16,000 inmates in England and Wales now ­identify themselves as Muslim after a 190 per cent rise in their numbers in 22 years due to sentencing and religious conversions behind bars. Almost a decade ago, Mr Acheson warned the Commons justice committee that “all the ingredients for radicalisation” in jails were present. Islamist gang culture, sometimes expressed in loyalty to the Muslim Brotherhood, mixes with ordinary criminality. Conversion to Islam behind bars is driven partly by the protection offered by gang membership in increasingly anarchic settings.