By exploiting the particular characteristics of variable structure control, a new approach is introduced to determine a system perturbation signal from the dynamics of the sliding mode. Perturbation feedforward compensation is then incorporated into the design of a control law that cancels the effects of system uncertainties and external disturbances. The robust control law ensures that the sliding mode is reached in finite time and with prescribed transient behaviour. Implementation aspects such as signal measurement uncertainty are also considered. Detailed computer simulation results are presented to demonstrate the effectiveness of the approach.