The growing number of terror attacks in Europe in recent time has made the concept of radicalization and violent extremism a worthy academic investigation, most especially Islamist motivated radicalization and extremism. The UK has witnessed the emergence of Islamists who have spent a period of jail term in prison and made effort to influence other inmates with their radical ideology. This research attempts to examine how prisons in the UK have been taken advantage of by Islamist inmates, whose goal is to influence many inmates to adopt their violent extremist orientation. It seeks to critically investigate why and how extremism and radicalisation in the UK prisons is possible. Combating prison radicalisation and extremism will assist to stop a form of security threat that this development poses, not only to the security of the UK in particular, but also to the global community in general.