A front underrun protection device for a heavy vehicle such as a truck, wherein the device comprises a beam configured to, when the device is arranged onto the vehicle, extend from side to side of the vehicle at a lower front position thereof and to absorb collision energy in case of e.g. a front collision between the vehicle and a passenger car, and wherein the beam is at least partly made of a plastic or polymeric material, that the beam comprises: - a housing beam member extending in a longitudinal direction of the beam and having at least one channel (34, 35) extending along the housing beam member, - at least one reinforcement beam member (4, 5) having an outer shape adapted to fit into the channel (34, 35) of the housing beam member, wherein the reinforcement beam member (4, 5) is arranged in the channel (34, 35) and arranged to extend along the housing beam member, wherein the reinforcement beam member (4, 5) is provided with a plurality of wall elements comprising transversal wall elements (51, 52) distributed along and extending across the reinforcement beam member (4, 5) and longitudinal wall elements (53, 53') extending in a longitudinal direction of the reinforcement beam member (4, 5) between the transversal wall elements (51, 52) wherein the housing beam member has a cross-section comprising a substantially U-shaped part with first and second substantially parallel outer panels (31, 32) connected by an inner panel that extends between the first and second outer panels (31, 32) in a plane substantially perpendicular to that of the outer panels (31, 32) so as to form the channel (34, 35) extending along the housing beam member at one side of the inner panel between the substantially parallel outer panels (31, 32) and characterized in that the housing member has a substantially H-shaped cross-section with the first and second substantially parallel outer panels (31, 32) connected by the inner panel that extends between the first and second outer panels (31, 32) in a plane substantially perpendicular to that of the outer panels (31, 32) so as to form first and second channels (34, 35) extending between the outer panels (31, 32) along the housing beam member at opposite sides of the inner panel. wherein the beam is at least partly made of a plastic or polymeric material, that the beam comprises: - a housing beam member extending in a longitudinal direction of the beam and having at least one channel (34, 35) extending along the housing beam member, - at least one reinforcement beam member (4, 5) having an outer shape adapted to fit into the channel (34, 35) of the housing beam member, - a housing beam member extending in a longitudinal direction of the beam and having at least one channel (34, 35) extending along the housing beam member, - at least one reinforcement beam member (4, 5) having an outer shape adapted to fit into the channel (34, 35) of the housing beam member, wherein the reinforcement beam member (4, 5) is arranged in the channel (34, 35) and arranged to extend along the housing beam member, wherein the reinforcement beam member (4, 5) is provided with a plurality of wall elements comprising transversal wall elements (51, 52) distributed along and extending across the reinforcement beam member (4, 5) and longitudinal wall elements (53, 53') extending in a longitudinal direction of the reinforcement beam member (4, 5) between the transversal wall elements (51, 52) wherein the housing beam member has a cross-section comprising a substantially U-shaped part with first and second substantially parallel outer panels (31, 32) connected by an inner panel that extends between the first and second outer panels (31, 32) in a plane substantially perpendicular to that of the outer panels (31, 32) so as to form the channel (34, 35) extending along the housing beam member at one side of the inner panel between the substantially parallel outer panels (31, 32) and characterized in that the housing member has a substantially H-shaped cross-section with the first and second substantially parallel outer panels (31, 32) connected by the inner panel that extends between the first and second outer panels (31, 32) in a plane substantially perpendicular to that of the outer panels (31, 32) so as to form first and second channels (34, 35) extending between the outer panels (31, 32) along the housing beam member at opposite sides of the inner panel.