The Centre for Education in the Criminal Justice System (CECJS) at UCL Institute of Education recognises that, in order to design a coherent prison education system, it is necessary to have an informed understanding of the current educational levels of the learners. Until recently, information available on the levels of literacy and numeracy skills of the prison population was considerably out of date, and therefore unhelpful to the current context. However, from this current and ongoing work we are beginning to gain an updated picture of skill levels. In November 2015, CECJS released an initial analysis into prisoners' basic skills levels based on the ‘mandatory assessments' in English and maths in 2014/15. This paper builds on that analysis using the official data gleaned from the 2014/15 Individualised Learning Record database, which is both consistent with, and supplements, the information from the prison education providers. The paper is structured to first validate the overall findings of the initial report regarding skills level of the incoming prison population. It examines how the assessment data is used by providers to inform placement of prisoners on appropriate basic skill courses, and analyses the progression trajectories of prisoners. It then details the performance of prisoners on basic English and maths courses, including their progression and achievement. The findings suggest a system that is failing to deliver education to its most vulnerable learners in prison. The majority of prisoners, including those with the lowest skills levels of English and maths, do not progress to higher levels and are insufficiently challenged. Of those prisoners enrolled on courses of study, only half complete and often only at levels lower than their previously assessed levels. The conclusion provides recommendations for policy makers and urgently calls for more research.