The diffusion of channelled beams determined from the continuum : odel is studied analytically taking into account a real discontinuous potential. In usual channelling conditions the diffusion due to the discontinuity considered here does not have a negligible effect on dechannelling. An analytical expression for the diffusion coefficient in dechannelling theory is given. We show that the diffusion considered here is one reason for the very weak temperature-dependence of planar-dechannelling.