  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 $d^{-1/2}$ 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 F\"orster mechanism, induced by the
electrostatic dipole-dipole interaction, is recovered.
