a method of fabricating a wind turbine blade tubular spar , wherein the tubular spar extends along a designated axis , is made of reinforced polymer material having fibers arranged in at least two directions , and a polymer matrix incorporating the fibers , and has two caps and two webs ; the method including the steps of : molding and cross - linking at least part of a cap or web in composite material ; molding and cross - linking an l - shaped structure , which has two opposite parallel flanges , and at least partly incorporates the previously molded , cross - linked cap or web of composite material ; repeating the above steps to mold and cross - link a further l - shaped structure ; and joining the two cross - linked , l - shaped structures , with the flanges positioned facing in pairs .