The use of an aqueous polymer dispersion including at least one amorphous polymer having a Tg varying between 55° C. and 175° C. or a semi-crystalline polymer having a melting point varying between 70° C. and less than 220° C., the weight concentration of the polymer in the dispersion varying between 5 and 50%, with the polymer particles having an average size of less than 10000 nm, as a binder for impregnating strands or strips of natural fibers, in particular flax fibers, with said polymer being incorporated in the core of the fiber bundle forming the strand or strip and thereby binding the fibers together.