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Quasi-2D behaviour of HTSCs: the consequence of the small effective thickness of superconducting layers

New evidence of quasi-two-dimensional (2D) behaviour in granular bulk high- samples is presented. Anomalously large differences between the mean-field critical temperature and the temperature of the Berezinsky - Kosterlitz - Thouless-like transition is found to be in good agreement with calculations based on the small inherent thickness of layers. Two successive mechanisms, quasiparticle and Josephson tunnelling, are used to explain the experimental data obtained on a large set of samples ranging from one unit cell to perfect single crystals. A simple phenomenological analysis based on the renormalization of 2D superfluid density of charge carriers through the thickness-restrictive mechanism of their mean free path is developed.