Abstract In the present work, a model for fibre–matrix debonding initiation under the biaxial loading condition (plane tension combined with out of plane shear) is proposed. The model is developed within the frame of Finite Fracture Mechanics and it is capable of predicting the critical stress value for the onset of debonding and the length of the initial defect. A parametric analysis is carried out to understand the influence of the main geometric and interface parameters on the critical debonding stress, which is found to be strongly dependent on the fibre radius. Satisfactory agreement with experimental data from the literature suggests that this approach, suitably adapted, might also be applied in the case of a unidirectional composite lamina, representing a very useful tool for modelling composite material behaviour under multiaxial loading conditions.