Aim: To design and assess the psychometric properties of a questionnaire to identify and measure factors that influence the identification of individuals at ultra-high risk for psychosis in primary care. It will inform the subsequent design of educational interventions to help general practitioners (GPs; primary care physicians) detect these individuals. Methods: The questionnaire was developed using the theory of planned behaviour (TPB). A semistructured discussion group elicited beliefs underlying GPs' motivations to detect these individuals and informed the construction of a preliminary 106-item questionnaire incorporating all constructs outlined in the TPB. A pilot phase followed, involving 79 GPs from 38 practices across 12 counties in England, to define the determinants of intention to identify these individuals. A psychometric model of item response theory was used to identify which items could be removed. Results: The final instrument comprised 73 items and showed acceptable reliability (α = 0.77-0.87) for all direct measures. Path analysis models revealed that all the TPB measures significantly predicted intention. Subjective norm, reflecting perceived professional influence, was the strongest predictor of intention. Collectively, the direct measures explained 35% of the variance of intention to identify individuals at ultra-high risk for psychosis, indicating a good fit with the TPB model. Conclusion: The TPB can be used to identify and measure factors that influence identification of individuals at ultra-high risk for psychosis in primary care.