Stiffened carbon-fibre-reinforced composite structures are extensively used in the aerospace industry for constructing aircraft wings, fuselage, and several other structural components. These structures are often prone to damage due to ageing, cyclic loading and impact. The wave propagation based structural health monitoring technique is widely used for identifying such damage in these structures. This paper presents the analysis of guided wave propagation in a repaired stiffened composite aircraft-wing panel, in order to understand the wave propagation phenomenon in such complex multi-layered structure. Towards this, a coordinated theoretical, numerical and experimental investigation has been carried out. The dispersion curves for the structure are theoretically obtained by using a fast and efficient semi-analytical model to study the dispersion characteristics of the propagating guided waves at the high-frequency range. An extensive finite element based numerical simulation of guided wave propagation in the sample structure is carried out in ABAQUS. Based on the theoretically obtained dispersion curves, different wave modes in the signals are effectively identified. It is observed that the presence of a localized patch repair region in the structure significantly influences the wave mode amplitudes and propagation velocities. Laboratory experiments are then conducted, in order to verify the numerical simulation results. A good agreement is noticed between the simulation and experimental results, in all the cases studied. A series of parametric study is also numerically carried out, in order to check the influence of repaired region size on the propagating guided wave modes in the structure.