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The British Swallowtail, Papilio machaon britannicus, is an iconic flagship for its unique but now restricted and fragmented fenland ecosystem in the UK's Norfolk Broads. Occurrence in 1 km(2) recording squares fell by 56% over the period of 1974-2014 and by 13% in 2005-2014, but the breeding populations, mainly confined to reserves, have increased in size. Climate change-induced sea-level rise and consequent seawater incursion into the Norfolk Broads represent a significant existential threat to the butterfly. Translocation to more secure fenland sites is recommended; several are being restored or recreated through stakeholder partnerships in East Anglia and Somerset. Well-researched introduction and management of the foodplant, Milk-parsley, is essential for such translocations to succeed. A better understanding is needed of the genetic structuring of the British Swallowtail populations using modern sequencing technologies, in particular to elucidate the significance of gene flow in relation to the viability of introductions to small or isolated sites. In a species in which hybridisation is commonplace across the Holarctic, the continued influx and future spread in Britain of the continental subspecies Papilio machaon gorganus may present a threat to the genetic integrity of subspecies britannicus, despite their differences in habitat and larval foodplants.