Bumblebees are ecologically and economically important, and some species have suffered dramatic population declines. The absence of morphological diagnostic characters for the identification of some species creates difficulties for basic ecological studies, and for conservation management. The widespread and commercially exploited bumblebee subgenus Bombus sensu stricto contains a cryptic species complex, known as the lucorum complex, which in Europe comprises B. lucorum, B. cryptarum and B. magnus. Little is known about these species and much of what has been reported is likely to have suffered from incorrect identification. Although the lucorum complex as a whole is common in Great Britain, we aimed to determine whether the populations of the individual species are vulnerable and require conservation action. Using genetic methods to distinguish them, we determined the geographic distribution and abundance of the lucorum complex species in Great Britain, and assessed the extent of niche differentiation between these species. We detected major differences in the geographic range, forage use and sensitivity to summer temperatures of the three species. Bombus lucorum was found to have the broadest distribution and diet, being present throughout mainland Great Britain, whereas B. cryptarum and B. magnus were absent from large areas of central and southern England. Bombus cryptarum and B. magnus were more likely to be found at sites with lower summer temperatures. Bombus magnus, the least abundant species, was found to exhibit an unusually tight biotope association with heathland habitat. This has conservation implications for B. magnus given the current threats to this habitat type.