High-field magnetisation measurements up to 38 T have been performed on single-crystalline Bi₂Sr₂CaCu₂O_{8+ delta} in order to investigate the irreversible behaviour associated with the flux line dynamics. It was found that the irreversible magnetisation vanishes asymptotically at high fields well below B_c2. As temperature is increased, above approximately 40 K, the magnetisation essentially become reversible. Critical currents J_c and volume pinning forces F_p were calculated and compared with the results of YBa₂Cu₃O_{7- delta}. The irreversibility line deduced from the present measurements as well as from other techniques is discussed in terms of a possible boundary, above which superconducting phase fluctuations play a dominant role for the dissipation.