The long history of the deliberate or accidental, human-mediated dispersal of flowering plants has led to the introduction of foreign genotypes of many species into areas of Europe hitherto occupied by potentially distinct native populations. Studies of the genetic and evolutionary consequences of such changes are handicapped by the difficulty of identifying the surviving native populations of many species in the absence of clear morphological differences. We investigated the relationship between putative native and introduced populations of the herbaceous perennial Meconopsis cambrica (Papaveraceae), as the isolated native populations of this species can be identified by historical and ecological evidence. In Britain the species is scarce and declining as a native but has become increasingly frequent in recent decades as a garden escape. Native populations from Spain and France were compared to native and introduced British populations using ITS and cpDNA sequences and AFLPs. Ten of the twelve British populations could be unambiguously assigned to native or introduced groups using cpDNA and AFLPs. The introduced plants appear to originate from the central and eastern Pyrenees rather than from native British sites. Two populations (including one previously considered native) cannot be classified unambiguously. There is unequivocal evidence for unidirectional gene-flow from native plants into two of the introduced populations and possible evidence for hybridization in three other sites (two native). The absence of biological barriers to hybridization suggests that the native and introduced gene pools of M. cambrica in Britain might eventually merge.