Abstract: Provided is a flame retardant and latent hardener composition including a blend of 30-100% (by weight based on total solids) of diammonium hydrogen phosphate, and dihydrogen phosphate, 0-50% (by weight based on total solids) of monoammonium. The flame-retardant/hardener composition is prepared as a solid blend or a liquid composition, the liquid composition being an aqueous composition including a liquid aqueous solution of the contents ranging from 25% w/w to 80% w/w. Methods for making flame retarded fiber boards using the composition as flame retarder and a hardener for the resin used in the production of the boards are also provided.