AbstractObjectiveThe aim of this study was to explore the impact social class biases may have on the diagnosis and treatment of clients by psychological and psychotherapeutic professionals working in the National Health Service (NHS) in Britain.MethodsPsychological and psychotherapeutic professionals working in the NHS were randomised to one of two video vignettes representing a psychological assessment session with either a ‘lower’ or ‘upper’ class client. The accent and dress of the client were varied to elicit class stereotypes. The video and study measures were accessed online via a link to a webpage. Participants completed measures of clinical reasoning, namely diagnosis, risk assessment and treatment, as well as measures of class self‐awareness.ResultsAmong the 156 practitioners who completed the measures there was little difference between the two groups for the clinical reasoning measures. The expectation was that participants believed a ‘lower class’ client was more likely to receive an ‘alcohol or substance misuse’ diagnosis (p = .002; d = 0.40). Unexpectedly, seeing a ‘lower class’ client led to significantly increased class self‐awareness scores, particularly those measures indicating reflection on personal conflicts relating to practitioners’ own social class and the impact such biases may have on their work.ConclusionsThere was little difference in clinical reasoning between the two class conditions. This may be due to client class cues priming the psychologist to reflect on their own potential class biases. Practitioners may benefit from further reflection and training on biases related to substance use and social class.