Amphibian declines around the world are a major conservation concern. Monitoring trends in abundance is therefore important. Exemplar models are required, with robust, easily assessed indicators of population size that have high and consistent detection probability and which can be quantified over large geographical scales. Natterjack toads Bufo calamita potentially fulfil these criteria. This amphibian is rare and increasingly endangered in the north European part of its range, including Britain. In this paper, we analyse data on population size (based on spawn string counts) and breeding success (toadlet production) collected over 20 years from all remaining natterjack sites in the UK, permitting for the first time an assessment of population trends of an endangered amphibian at the national scale. State-space models, which account for observation error, were developed to estimate population trends and to assesss the effects of conservation management. Between 1990 and 2009, the British population of B. calamita was approximately stable as judged by spawn string counts and broadly confirmed by state-space modelling, although this indicated that continuing small decline was more probable than stability. Empirical and model analyses also demonstrated that population growth rate was influenced positively by frequency of breeding success (toadlet production) and by grazing of the terrestrial habitat by domestic livestock. The implications of these findings for future conservation management of B. calamita are discussed.