OBJECTIVES: To compare the clinical and cost-effectiveness of face-to-face (FTF) with over-the-telephone (OTT) delivery of low intensity cognitive behavioural therapy. DESIGN: Observational study following SROBE guidelines. Selection effects were controlled using propensity scores. Non-inferiority comparisons assessed effectiveness. SETTING: IAPT (improving access to psychological therapies) services in the East of England. PARTICIPANTS: 39,227 adults referred to IAPT services. Propensity score strata included 4,106 individuals; 147 pairs participated in 1:1 matching. INTERVENTION: Two or more sessions of computerised cognitive behavioural therapy (CBT). MAIN OUTCOME MEASURES: Patient-reported outcomes: Patient Health Questionnaire (PHQ-9) for depression; Generalised Anxiety Disorder questionnaire (GAD-7); Work and Social Adjustment Scale (WSAS). Differences between groups were summarised as standardised effect sizes (ES), adjusted mean differences and minimally important difference for PHQ-9. Cost per session for OTT was compared with FTF. RESULTS: Analysis of covariance controlling for number of assessments, provider site, and baseline PHQ-9, GAD-7 and WSAS indicated statistically significantly greater reductions in scores for OTT treatment with moderate (PHQ-9: ES: 0.14; GAD-7: ES: 0.10) or small (WSAS: ES: 0.03) effect sizes. Non-inferiority in favour of OTT treatment for symptom severity persisted as small to moderate effects for all but individuals with the highest symptom severity. In the most stringent comparison, the one-to-one propensity matching, adjusted mean differences in treatment outcomes indicated non-inferiority between OTT versus FTF treatments for PHQ-9 and GAD-7, whereas the evidence was moderate for WSAS. The per-session cost for OTT was 36.2% lower than FTF. CONCLUSIONS: The clinical effectiveness of low intensity CBT-based interventions delivered OTT was not inferior to those delivered FTF except for people with more severe illness where FTF was superior. This provides evidence for better targeting of therapy, efficiencies for patients, cost savings for services and greater access to psychological therapies for people with common mental disorders.