OBJECTIVE: Features of the childcare environment may influence children's in-care physical activity (PA). We assessed the association between UK preschool care-provider, environmental and policy factors and 3-4-year-olds' average daily in-care sedentary behaviour (SED) and PA. METHODS: In 2013, we used accelerometers to measure the in-care SED/ PA of 201 3-4-year-old children (51% female) in 30 preschools in Cambridgeshire, UK, (average wear time: (mean ± SD) 4.2 ± 1.3 week-days). We assessed the childcare environment using the Environment and Policy Assessment and Observation tool; demographic and carer information was taken from questionnaires. We used three-level mixed-effects regression analyses (adjusted for sex, in-care time and travel mode to care) to determine the association between childcare factors and children's in-care average daily minutes/hour spent SED, in light PA (LPA) and in moderate-to-vigorous PA (MVPA). RESULTS: Children spent 5.6 ± 2.5 h in care per day on average; clustering of PA within preschools was limited (ICCs: 0.003-0.05). Fully adjusted models showed that active opportunities were positively associated with children's in-care SED. No associations with in-care LPA and MVPA were observed. CONCLUSION: Few care-provider, environmental and policy factors were associated with children's in-care activity. UK childcare policies advocating child-driven play, moving freely indoors and outdoors, may be more conducive to individual children's PA.