1. Horizon scanning is an essential tool for environmental scientists if they are to contribute to the evidence base for Government, its agencies and other decision makers to devise and implement environmental policies. The implication of not foreseeing issues that are foreseeable is illustrated by the contentious responses to genetically modified herbicide-tolerant crops in the UK, and by challenges surrounding biofuels, foot and mouth disease, avian influenza and climate change. 2. A total of 35 representatives from organizations involved in environmental policy, academia, scientific journalism and horizon scanning were asked to use wide consultation to identify the future novel or step changes in threats to, and opportunities for, biodiversity that might arise in the UK up to 2050, but that had not been important in the recent past. At least 452 people were consulted. 3. Cases for 195 submitted issues were distributed to all participants for comments and additions. All issues were scored (probability, hazard, novelty and overall score) prior to a 2-day workshop. Shortlisting to 41 issues and then the final 25 issues, together with refinement of these issues, took place at the workshop during another two rounds of discussion and scoring. 4. We provide summaries of the 25 shortlisted issues and outline the research needs. 5. We suggest that horizon scanning incorporating wide consultation with providers and users of environmental science is used by environmental policy makers and researchers. This can be used to identify gaps in knowledge and policy, and to identify future key issues for biodiversity, including those arising from outside the domains of ecology and biodiversity. 6. Synthesis and applications. Horizon scanning can be used by environmental policy makers and researchers to identify gaps in knowledge and policy. Drawing on the experience, expertise and research of policy advisors, academics and journalists, this exercise helps set the agenda for policy, practice and research.