Modelling and prediction of the environmental degradation of fibre reinforced plastics (FRP) has been hindered by the complexity of the process. Published works are limited to effects and mechanism characterisation or partial models, most of the time empirical. In this article, an analytical approach is presented which resolves the degradation process into only three components: the chemical link density variation, the cohesion force variation and the stress state modification. The first two are referred to as chemical and physical degradation. Based on material science theories, the analysis demonstrates that in a constant environment an exponential function correlates the chemical and physical degradation to the environmental factors. It is also shown that the chemical and physical degradation rate in a real service environment can be determined in a laboratory in a constant environment based only on the variation of chemical link density. Laboratory experiments show that the model correlates excellently with the degradation process.