Unsaturated polyester (UP) based glass fibre-reinforced polymer composites (GFRCs) are extensively used in marine applications due to their high strength-to-weight ratio, low cost, good corrosion and chemical resistance and low maintenance requirements. However, despite all these advantages, the poor fire resistance of GFRCs is the major limiting factor for their use in certain applications such as passenger vessels, naval ships, submarines etc., where fire safety is an important issue. In this research, the chosen methodology to enhance the flame retardancy of unsaturated polyester resin is to blend it with an inherently flame retardant resin so that the resultant composition may effectively gain the favourable property of each component. The inherently flame retardant resin could be phenolic (PH) or melamine formaldehyde (MF). The main problem of blending UP and PH resin is their incompatibility due to their different curing mechanisms, which can lead to phase separation and hence, poor mechanical properties of the composite produced. In order to make them compatible, functionalised/modified resins have been used. In this PhD work, co-blending of UP with following resins have been studied: a) unfunctionalised/functionalised resole phenolic resins (Res-PH): four resins chosen were water-based resole (PH1), ethanol-soluble (PH2), epoxy-functionalised (PH3), and allyl-functionalised (PH4); b) flame retardants (resorcinol bis (diphenyl phosphate (RDP), bisphenol-A bisdiphenyl phosphate (BADP) and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)) added to Res-PH resins; c) modified novolac phenolic: methacrylated novolac (M-Nov) resin; and d) melamine formaldehyde resin (MF). The curing behaviour and compatibility of UP, other resins and their blends have been studied by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). These results showed that the blends of UP with the functionalised phenolic resins are chemically compatible and can be co-cured. Based on the successful establishment of curing conditions, plaques of resins have been cast and cured. The most compatible polymer blend systems (UP with functionalised/ modified phenolic resin) such as UP/PH4 and UP/M-Nov systems showed DMTA results with single glass transition temperature (Tg) and no phase separation in the scanning electron microscopic (SEM) images, while incompatible systems showed signs of phase separation. Limiting oxygen index (LOI) measurements and cone calorimetry at 50kW/m2 heat flux have shown that fire performance of the functionalised Res-PH resins and their blends with UP is inferior than that from the un-functionalised Res-PH resin, but still significantly better than that of the UP. The blended resins with added FRs showed the similar trend i.e., FRs are, least effective in un-functionalised (UP/PH2 blends) blends compared with functionalised blends (UP/PH3 and UP/PH4). To understand this behaviour, thermal analyses using thermo gravimetric analysis (TGA) coupled with infrared spectroscopy of volatile degradation products have been used on all resins and their blends, based on which, mechanisms of their decomposition and interactions are proposed, and the effects of these on flammability are discussed. UP/inherently flame retardant resin blends show improved thermal stability and improved flame retardancy than that of UP due to the char formation of inherently flame retardant resin component of the bend, whereas the UP resin decomposes into combustible volatiles, which burn. The glass fibre-reinforced composites (GFRCs) from co-blended matrices were prepared by hand lay-up method. The flammability of the composite samples was assessed by means of cone calorimetry and UP-94 tests. The GFRCs from co-blended resins showed better flame retardancy in terms of significant reductions in peak heat release rate (PHRR) and improved char residue. Finally the mechanical properties of the composite laminates have been studied by flexural, impact and tensile tests. The GFRCs from blends of UP with non-functionalised phenolic (UP/PH2) with and without FRs showed poor mechanical properties due to poor compatibility between UP and PH2, whereas, the GFRC from UP with functionalised phenolic resin, PH3-epoxy functionalised, PH4-allyl functionalised with and without FRs and UP/M-Nov, showed little deterioration in mechanical properties.