UK cancer deaths remain high; primary care is key for earlier cancer diagnosis as half of avoidable delays occur here. Improvement is possible through lower referral thresholds, better guideline adherence, and better safety netting systems. Few interventions target whole practice teams. We developed a novel whole practice team intervention to address this. To test the feasibility and acceptability of a novel, complex behavioural intervention 'ThinkCancer!' for assessment in a subsequent Phase III trial. Pragmatic, superiority pilot RCT with an embedded process evaluation and feasibility economic analysis in Welsh general practices. Clinical outcome data were collected from practices (the unit of randomisation). Practice characteristics and cancer safety netting systems were assessed. Individual practice staff completed evaluation and feedback forms, and qualitative interviews. The intervention was adapted and refined. Trial recruitment and workshop deliveries took place between March 2020 to May 2021. Trial progression criteria for recruitment, intervention fidelity and routine data collection were met. Staff-level fidelity, retention and individual level data collection processes were reviewed and amended. Interviews highlighted positive participant views on all aspects of the intervention. All practices set out to liberalise referral thresholds appropriately, implement guidelines, and address safety netting plans in detail. 'ThinkCancer!' appears feasible and acceptable; the new iteration of the workshops was completed, and the Phase III trial has been funded to assess the effectiveness and cost effectiveness of this novel professional behaviour change intervention. Delivery at scale to multiple practices will likely improve fidelity and reach. [Abstract copyright: Copyright © 2024, The Authors.]