Introduction Dietary analyses utilising visual methods to identify stomach and faecal contents have shown that urban red foxes (Vulpes vulpes) in the United Kingdom (UK) consume human-derived (anthropogenic) food to varying degrees. Anthropogenic foods have been implicated in poor health outcomes for synanthropic species that consume them, therefore it is critical to examine the degree of such foods in the UK fox diet. Methods We analysed the stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) of whiskers collected from 93 foxes from across the UK to determine: 1) the viability of stable isotope analysis (SIA) for distinguishing between anthropogenic and natural food consumption in foxes; 2) the degree to which urban foxes consume anthropogenic food compared to rural foxes using a Bayesian mixing model; 3) whether factors such as sex, age and season predict individual levels of consumption of human-derived food in foxes. Main results 1) Urban fox diet was significantly different to rural foxes: urban foxes demonstrated significantly higher δ13C and lower δ15N, a pattern consistent with anthropogenic food consumption. 2) We estimated that food provided either directly or indirectly by humans contributed around 35% of the urban fox diet compared to approximately 6% in the rural fox diet. 3) Across rural and urban foxes combined, there were significant isotopic differences between males and females, with females demonstrating higher δ13C and lower δ15N. 4) No differences in δ13C and δ15N between subadults and adults were observed. 5) Values of δ15N differed significantly between samples from the summer and the winter, where summer had the highest δ15N and winter the lowest. Additionally, winter had the highest δ13C values of the seasons, although this difference was not significant. Conclusion Potential negative outcomes of anthropogenic food consumption are likely to disproportionately impact females than males and urban-dwelling foxes than rural foxes.