The present disclosure relates to a method of manufacturing a security sheet (45) comprising a plastic substrate (20). The plastic substrate comprises opposing first and second outer surfaces (21, 22), a first concealing region (25) extending between the first and second outer surfaces and a laser markable region (29) at least partially located between the first concealing region and the second outer surface. Laser radiation (36) is directed from the first outer surface, through the first concealing region and onto the laser markable region to form at least one data element (40) in the laser markable region such that the at least one data element is hidden from view by the first concealing region when the first outer surface is viewed by the naked eye in reflected light.