The English language proficiency and academic literacy needs of increasing numbers of incoming students from diverse language and educational backgrounds constitute one of the major challenges faced by English Medium of Instruction (EMI) universities, both in English speaking 'inner circle' countries (Kachru, 1985) and elsewhere, as a consequence of the globalisation of higher education. Unprecedented pressure on universities to address the manifold academic language needs of diverse cohorts of students has resulted in a move to fashion academic English language development mechanisms, known as English for academic purposes (EAP) programmes in the UK. A growing body of research dedicated to academic English provision has thus emerged. Despite the burgeoning interest in this area of enquiry, however, research in the UK, unlike other native English-speaking contexts (e.g. Australia), has primarily focused on pre-entry EAP programmes - referred to in this context as pre-sessional courses - rather than post-entry EAP provision, normally referred to as in-sessional courses. This study therefore examined the characteristics and efficacy of current in-sessional programmes on offer across British universities and the principles underlying the design and delivery of such programmes. It also evaluated the effectiveness of the provision and those factors determining its efficacy. To this end, a mixed-method approach consisting of a large-scale online survey and a series of follow-up interviews was employed, in which three primary stakeholder groups participated: students, subject specialists and in-sessional staff. The findings suggest that, despite some variation, the provision tends to constitute primarily non-credit-bearing English for general academic purposes courses, predominantly targeted at non-native speakers of English, and offered centrally via different units across institutions. In terms of effectiveness, the results showed that the provision is perceived as less effective in terms of adequacy, relevance, and needs analysis respectively. In addition, new evaluation sub-criteria emerged, other than those reflected in best practice principles literature on EAP, based on stakeholders' perceptions of the provision. These findings contribute to the existing literature on in-sessional EAP provision by providing a more comprehensive understanding of what current in-sessional provision across British universities looks like, and identifying those factors which determine its efficacy, based on both best practice principles in EAP and stakeholders' perceptions.