The shear lag effect in composite twin-girder decks is analysed by considering the main prestressing techniques, namely support settlements, cables inside the concrete slab and external slipping cables. The theoretical model, which includes concrete creep behaviour and the shear connection flexibility, describes the non-uniform stress distribution on the slab by means a warping function taking into account the real position of the two beams. Numerical applications are carried out on a realistic bridge deck by separately considering the prestressing systems listed above. The effective slab width describing the shear-lag effect on the cross section and its evolution in time are evaluated for each kind of prestressing and compared with those given by the method suggested by the European Code ENV1994-2.