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Background: Previous research has illustrated a drift in the fidelity of behaviour change techniques (BCTs) during the design of the pilot NHS England Low‐Calorie Diet (NHS‐LCD) Programme. This study evaluated a subsequent domain of fidelity, intervention delivery. Two research questions were addressed: (1) To what extent were BCTs delivered with fidelity to providers programme plans? (2) What were the observed barriers and facilitators to delivery? Methods: A mixed‐methods sequential explanatory design was employed. Remote delivery of one‐to‐one and group‐based programmes were observed. A BCT checklist was developed using the BCT Taxonomy v1; BCTs were coded as present, partially delivered, or absent during live sessions. Relational content analysis of field notes identified observed barriers and facilitators to fidelity. Results: Observations of 122 sessions across eight samples and two service providers were completed. Delivery of the complete NHS‐LCD was observed for five samples. Fidelity ranged from 33% to 70% across samples and was higher for group‐based delivery models (64%) compared with one‐to‐one models (46%). Barriers and facilitators included alignment with the programme's target behaviours and outcomes, session content, time availability and management, group‐based remote delivery, and deviation from the session plan. Conclusions: Overall, BCTs were delivered with low‐to‐moderate fidelity. Findings indicate a dilution in fidelity during the delivery of the NHS‐LCD and variation in the fidelity of programmes delivered across England. Staff training could provide opportunities to practice the delivery of BCTs. Programme‐level changes such as structured activities supported by participant materials and with sufficient allocated time, might improve the delivery of BCTs targeting self‐regulation. [ABSTRACT FROM AUTHOR]