<p>Poplar wood (<i>Populus alba</i> L.) was modified using a fully bio-based flame-retardant system based on furfuryl alcohol (FA) and ammonium phytate (AMP), catalysed by phosphoric acid (H₃PO₄). The treatment enables in-situ polymerization of FA to form a cross-linked poly(furfuryl alcohol) (PFA) network that immobilizes the nitrogen-phosphorus flame retardant within the wood structure. The modified wood exhibited a weight% gain of ≈ 22% and a density increase from 0.39 to 0.50&#xa0;g cm^− 3. FTIR and SEM–EDX confirmed incorporation of phosphorus- and nitrogen-containing species throughout the wood matrix. Fire testing showed a substantial improvement in flame retardancy, including an increase in limiting oxygen index from 18% to 28%, a strong reduction in heat release and smoke production, and a more than six-fold increase in residual char. Thermogravimetric analysis demonstrated earlier dehydration and enhanced char formation, consistent with a synergistic condensed-phase mechanism of the P–N system. Importantly, the treatment exhibited high leach resistance (&gt; 90%), and the enhanced fire performance was largely retained after a 14-day leaching procedure, confirming durable fixation of the flame-retardant components. The combined FA + AMP+H₃PO₄ system also reduced water uptake and improved surface hardness, while maintaining acceptable mechanical performance. The results demonstrate that phosphoric acid can act as a dual-function catalyst and supplementary phosphorus source, enabling a low-concentration, renewable, and durable flame-retardant modification of fast-growing poplar wood. This approach provides a sustainable pathway for producing multifunctional, high-performance lignocellulosic materials.</p>