Thermoplastic composites demand constant improvements in fire retardant and mechanical properties to fulfil their full market potential, especially in demanding sectors such as rail, aerospace and infrastructure, where fire performance is critical. The aim of this work is to understand the effect of reinforcing fibres on the flammability of polypropylene–glass (P-G) composites and the means of improving their fire performance in a cost-effective manner. A number of P-G composites with 0%, 10% and 20% (w/w) glass fibres were prepared using short length glass fibres. The effect of fibre content on the thermal stability, flammability and mechanical performance of the P-G composites without and in the presence of conventional fire retardants was studied. It was observed that while the presence of glass fibre lowered down the limiting oxygen index value of the composite, the rate of flame spread in a UL-94 equivalent test was also lowered. The reduction in limiting oxygen index is due to the fact that glass fibre reduces the melt dripping behaviour of polypropylene and does not let the polymer (polypropylene) move away from flame, which then burns. Cone calorimetric study indicated that the presence of glass fibre reduces the overall flammability of the composite laminate. Copyright © 2011 John Wiley & Sons, Ltd.