Method of manufacturing a wind turbine blade

A method of manufacturing a wind turbine blade, the blade having a profiled contour including a pressure side and a suction side, and a leading edge and a trailing edge with a chord having a chord length extending therebetween, the wind turbine blade extending in a spanwise direction between a root end and a tip end, said method comprising:

• providing a mould (66);
• arranging one or more layers of fibre material in the mould for providing a skin element (70);
• arranging a preform (76) on at least part of the one or more layers of fibre material for providing a reinforcing element;
• injecting the one or more layers of fibre material and the preform (76) with a curable resin;
• and
• curing the resin,
• wherein the preform (76) is impregnated with a curing promoter such that the concentration of curing promoter varies spatially within the preform (76).