A serrated panel for a wind turbine blade, wherein the panel is configured to be attached to the trailing edge of a blade to form a plurality of serrations at the trailing edge of the blade, wherein the serrated panel comprises: - a base part for attaching the panel to the trailing edge of the blade, the base part having - a first longitudinal end for arrangement nearest a tip end of the wind turbine blade, - a second longitudinal end for arrangement nearest a root end of the wind turbine blade, - a first side for arrangement nearest a leading edge of the blade, - a second side for arrangement farthest from the leading edge of the blade, - an attachment surface for attaching to an exterior surface of the wind turbine blade, and - an exterior surface facing away from the exterior surface of the wind turbine blade, when the serrated panel is attached to the wind turbine blade, and - a plurality of serrations extending from the second side of the base part, wherein the serrations comprise a base proximal to second side of the base part, and an apex distal to the second side of the base part with a notional line extending from a midpoint of the base to the apex, wherein the serrations comprises a shape and are mutually spaced so that a slit is formed between adjacent serrations, wherein each of the slits comprises a first side wall at a first adjacent serration and a second side wall at a second adjacent serration, characterised by a connecting surface extending between the first side wall and the second side wall, wherein the connecting surface is substantially flat or flattened and comprises a first rounded surface at the first sidewall and a second rounded surface at the second side wall. - a base part for attaching the panel to the trailing edge of the blade, the base part having - a first longitudinal end for arrangement nearest a tip end of the wind turbine blade, - a second longitudinal end for arrangement nearest a root end of the wind turbine blade, - a first side for arrangement nearest a leading edge of the blade, - a second side for arrangement farthest from the leading edge of the blade, - an attachment surface for attaching to an exterior surface of the wind turbine blade, and - an exterior surface facing away from the exterior surface of the wind turbine blade, when the serrated panel is attached to the wind turbine blade, and - a first longitudinal end for arrangement nearest a tip end of the wind turbine blade, - a second longitudinal end for arrangement nearest a root end of the wind turbine blade, - a first side for arrangement nearest a leading edge of the blade, - a second side for arrangement farthest from the leading edge of the blade, - an attachment surface for attaching to an exterior surface of the wind turbine blade, and - an exterior surface facing away from the exterior surface of the wind turbine blade, when the serrated panel is attached to the wind turbine blade, and - a plurality of serrations extending from the second side of the base part, wherein the serrations comprise a base proximal to second side of the base part, and an apex distal to the second side of the base part with a notional line extending from a midpoint of the base to the apex, wherein