A wind turbine blade is described having a serrated trailing edge. Flow straightening vanes are provided on the serrations, to prevent a lateral or side flow over the edges of the serrations, which are preferably provided at incidence to the flow over the wind turbine blade. The vanes can be formed integrally with the serrations, or attached to existing serrations as a retrofit solution. The serrations with the vanes can be provided as a trailing edge panel for attachment to the trailing edge of an existing wind turbine blade.