This article examines whether there is a collective climate action intention-behaviour gap amongst the British public, and the role of social identification in bridging the gap. Using a nationally representative survey (n=1100), we measured participant’s intentions to take collective climate action (such as volunteering, sharing climate change information with friends, protesting), and actual behavioural engagement with Extinction Rebellion (XR), a large-scale environmental movement. We also measured psychological factors based on the Social Identity Model of Pro-Environmental Action (SIMPEA) model, such as social identification, and socio-demographic characteristics. We found 50% of the participants were inclined abstainers, i.e., they expressed intentions to take collective climate action despite not having performed any actions with XR so far. Based on a moderated mediation model, we found that social identification predicted behavioural engagement with XR but did not moderate the collective climate action intention-behaviour gap. We discuss why this may be the case, including the role of public perceptions of how effective and disruptive XR's actions are.