BackgroundEarly mobilisation in critical care is recommended within clinical guidance; however, mobilisation prevalence across the UK is unknown. The study aimed to determine the proportion of patients mobilised out of bed within 48–72 h, to describe their physiological status, and to compare this to published consensus safety recommendations for out-of-bed activity.MethodsA UK cross-sectional, multi-centre, observational study of adult critical care mobility practices was conducted. Demographic, physiological and organ support data, mobility level, and rationale for not mobilising out of bed, were collected for all patients on 3rd March 2022. Patients were categorised as: Group 1—mobilised ICU Mobility Scale (IMS) ≥ 3; Group 2—not-mobilised IMS < 3 with physiological reasons; or Group 3—not-mobilised IMS < 3 with non-physiological barriers to mobilisation. Rationale for the decision to not mobilise was collected qualitatively. Regression analysis was used to compare the physiological parameters of Group 1 (mobilised) versus Group 2 (not-mobilised with physiological reasons). Patients were stratified as ‘low-risk’, ‘potential-risk’ or ‘high-risk’ using published risk of adverse event ratings.ResultsData were collected for 960 patients across 84 UK critical care units. Of these 393 (41%) mobilised, 416 (43%) were not-mobilised due to physiological reasons and 151 (16%) were not mobilised with non-physiological reasons. A total of 371 patients had been admitted for ≤ 3 days, of whom 180 (48%) were mobilised, 140 (38%) were not mobilised with physiological reasons, and 51 (14%) were not mobilised with non-physiological reasons. Of the 809 without non-physiological barriers to mobilisation, 367 (45%) had a low risk of adverse event rating and 120 (15%) a potential risk, of whom 309 (84%) and 78 (65%) mobilised, respectively. Mobility was associated with a Richmond Agitation-Sedation Scale of − 1 to + 1, lower doses of vasoactive agents, a lower inspired oxygen requirement.ConclusionAlthough only 40% of patients mobilised out of bed, 89% of those defined ‘low-risk’ did so. There is significant overlap in physiological parameters for mobilisation versus non-mobilisation groups, suggesting a comprehensive physiological assessment is vital in decision making rather than relying on arbitrary time points.Clinical Trials registration:NCT05281705 Registered March 16, 2022. Retrospectively registered.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13054-023-04508-4.