A strip of fibre-reinforced polymeric material for a longitudinal reinforcing structure of a wind turbine blade, the strip having substantially flat upper and lower surfaces and extending longitudinally between first and second transverse edges, wherein an end region of the strip tapers in thickness towards the first transverse edge, and wherein one or more slots are defined in the tapered end region, the or each slot extending longitudinally from the first transverse edge of the strip into the tapered end region.