This study aimed to identify strategies to increase the capacity for general practice training in London. All accredited general practitioner trainers (n = 351) in the London region were sent a questionnaire about current and past training activity and future intentions; 274 (78%) responded. A follow-up qualitative study of 33 general practitioners and other stakeholders explored attitudes towards barriers to training for active and inactive trainers and those not accredited. Key findings were a view that there was an intrinsic value to involvement in training, and the identification of common areas of concern. For current trainers, there was widespread concern about increasing 'bureaucratisation' of training, especially the allocation process. For those not accredited, the major constraints on involvement were space, competing professional commitments and, for some, staff and financial resources. Findings suggest that, within the pool of known accredited trainers, activity rates cannot be increased by a large margin, as a proportion of inactivity each year is inevitable. To increase training capacity, the key route is to develop more training practices and train more trainers. This will require targeted support, in particular paying locum costs to allow protected time to train and addressing the urgent need for increased space of many practices in London. At the same time there should be continued support and encouragement for current trainers.