Background: Informal care plays a crucial role in the social care system in England and is increasingly recognised as a cornerstone of future sustainability of the long-term care (LTC) system. This paper explores the variation in informal care provision over time, and in particular, whether the considerable reduction in publicly-funded formal LTC after 2008 had an impact on the provision of informal care. Methods: We used small area data from the 2001 and 2011 English censuses to measure the prevalence and intensity (i.e. The number of hours of informal care provided) of informal care in the population. We controlled for changes in age structure, health, deprivation, income, employment and education. The effects of the change in formal social care provision on informal care were analysed through instrumental variable models to account for the well-known endogeneity. Results: We found that informal care provision had increased over the period, particularly among high-intensity carers (20+ hours per week). We also found that the reduction in publicly-funded formal care provision was associated with significant increases in high-intensity (20+ hours per week) informal care provision, suggesting a substitutive relationship between formal and informal care of that intensity in the English system.