The present invention concerns a novel sustained-release oral formulation of morphine sulphate in the form of microgranules. Each microgranule comprises a neutral support grain coated with an active layer and with a sustained-release layer, characterized in that the sustained-release layer contains a copolymer of methacrylic acid and of methyl methacrylate ester, the relative proportion of the free carboxyl groups and of the ester groups of which is equal to 0.5 approximately, and a silica exhibiting a hydrophobic character. The present invention also concerns a process for preparing these microgranules which is carried out entirely in aqueous medium by emplacing on neutral support grains.