The paper presents a new method of designing decentralised load-frequency regulators for interconnected power systems. Within the framework of this method, the interconnected mutliarea power system is decomposed into several subsystems, each of which is controlled separately by a decentralised regulator. Each subsystem consists of one area and its external equivalent in a simplified form. A decentralised control law for the study area is introduced by using a quadratic performance index. Feedback gains of the decentralised regulator, which minimise the index, are determined by a Newton-Raphson iterative algorithm. The proposed method is applied to an interconnected longitudinal 4-area system, and the effects of the proposed regulator are examined by digital simulations and associated sensitivity analysis of the system. Furthermore, a suitable means for preventing excessive control action is also considered involving a significant system nonlinearity, i.e. some generation rate constraint.