ObjectivesPulmonary rehabilitation (PR) is a highly effective, recommended intervention for patients with chronic obstructive pulmonary disease (COPD). Using behavioural theory within mixed-methods research to understand why referral remains low enables the development of targeted interventions in order to improve future PR referral.DesignA multiphase sequential mixed-methods study.SettingUnited Kingdom (UK).Participants252 multiprofessional primary healthcare practitioners (PHCPs).MeasuresPhase 1: semistructured interviews. Phase 2: a 54-item paper and online questionnaire, based on the Theoretical Domains Framework (TDF). Content and descriptive analysis utilised. Data mixed at two points: instrument design and interpretation.Results19 PHCPs took part in interviews and 233 responded to the survey. Integrated results revealed that PHCPs with a post qualifying respiratory qualification (154/241; 63.9%) referred more frequently (91/154; 59.1%) than those without (28/87; 32.2%). There were more barriers than enablers for referral in all 13 TDF domains. Key barriers included: infrequent engagement from PR provider to referrer, concern around patient’s physical ability and access to PR (particularly for those in work), assumed poor patient motivation, no clear practice referrer and few referral opportunities. These mapped to domains: belief about capabilities, social influences, environment, optimism, skills and social and professional role. Enablers to referral were observed in knowledge, social influences memory and environment domains. Many PHCPs believed in the physical and psychological value of PR. Helpful enablers were out-of-practice support from respiratory interested colleagues, dedicated referral time (annual review) and on-screen referral prompts.ConclusionsReferral to PR is complex. Barriers outweighed enablers. Aligning these findings to behaviour change techniques will identify interventions to overcome barriers and strengthen enablers, thereby increasing referral of patients with COPD to PR.