Fibre reinforced polymer (FRP) composites are used in various structural engineering applications due to their advantages over traditional materials, such as high strength, durability and low self-weight. However, due to their polymeric nature, their mechanical properties are reduced at elevated temperature (ET), raising safety concerns under service conditions or accidental fire exposure. Therefore, understanding (and being able to predict) the behaviour of FRP composites under ET is essential to ensure the safe and economic design of FRP structures. This paper presents experimental and analytical studies about the variation of the longitudinal elastic modulus of glass-FRP (GFRP) laminates with different fibre architectures when exposed to ET (up to 300 °C). The Classical Laminate Theory (CLT), originally developed without temperature considerations, is extended to encompass the variation with temperature of the laminate stiffness properties. The proposed model adapts the original formulation by incorporating the temperature-induced degradation of each individual constituent (reinforcing fibres and resin matrix), affecting the mechanical properties of the plies and, consequently, of the laminate. For that, the resin degradation with temperature was experimentally determined and the temperature-dependent mechanical properties of GFRP laminates with different fibre architectures (previously determined by the authors) was considered. The degradation of the fibre reinforcement with temperature was calibrated using data from unidirectional laminates, and the model was validated for other reinforcement configurations (bi-directional, off-axis and multiaxial), yielding reasonably accurate predictions. The results highlight the possibility of considering the individual degradation of the ply constituents in predicting the longitudinal elastic modulus of FRP laminates at ET.