Abstract The diffractive dissociation of photons with virtuality Q 2 < 0.7 GeV 2, γ* p → Xp, has been studied in ep interactions with the ZEUS detector at HERA. For a subsample of the data, the scattered proton was measured with the Leading Proton Spectrometer. The dependence of the diffractive cross section on the photon-proton centre-of-mass energy is used to determine the Pomeron intercept. The diffractive structure function F D 2 is measured and its dependence on Q 2 studied. At the small values of Q 2 considered, F D 2 exhibits a strong Q 2 dependence, a result at variance with what has been observed at higher Q 2 values.