This paper presents a method for the analysis of planar multilayered waveguide-fed antennas. The method combines mixed-potential integral equations (for laterally open regions) and modal field integral equations (for laterally closed regions) with a seamless transition between the two domains. The method has been implemented in a numerical tool and the simulation results of two waveguide-fed microstrip structures have been presented. The results are in good agreement with both measurements and simulations obtained with other commercial electromagnetic tools. Comparisons in terms of memory occupation and simulation time have also been performed.