A flat reinforcer has the shape of a multicomposite strip defining three main perpendicular directions, the axial direction (X), the transverse direction (Y) and the radial direction (Z), having a width WR measured along the Y direction of between 2 and 100 mm and having a thickness TR measured along the Z direction of between 0.1 and 5 mm, the aspect ratio WR/TR being greater than 3. The multicomposite strip comprises at least a plurality of monofilaments made of composite material, orientated along the X direction, comprising filaments of a mineral material (101) embedded in a thermoset resin (102), the glass transition temperature of which, denoted Tg1, is greater than 70° C. The plurality of monofilaments are coated in a layer of thermoplastic material (12); and all or some of these monofilaments are flat, of cross section having a width WM measured along the Y direction and having a thickness TM measured along the Z direction, with an aspect ratio WM/TM greater than 1.5.