We study the system of two quantum dots lying on the central plane of a planar semiconductor microcavity. By solving the full Maxwell problem, we demonstrate that the rate of resonant excitation transfer between the two dots decays as d(-12) as a function of the distance d at long distance. This very long-range mechanism is due to the leaky and guided modes of the microcavity, which act as effective radiative transfer channels. At short distance, the d(3) dependence of the Forster mechanism, induced by the electrostatic dipole-dipole interaction, is recovered.