A wind turbine blade which is at least 45 m long. The blade has a main axis in the lengthwise direction from root to tip and an aerodynamic shell surrounding a spar which extends in a lengthwise direction along a substantial proportion of the blade at least a portion of the half of the shell closest to the tip. The blade is reinforced by high stiffness fibers having a Young's modulus to density ratio of greater than 50 GPa/(g·cm−3), and an electrical resistivity of greater than 1010 Ω·cm. The fibers are biaxial with a first axis angled with respect to the main axis at an acute angle and a second axis angled with respect to the main axis at an acute angle in the opposite sense to the first angle.