Competition in the electric power industry urges utilities to not only reduce the investment costs, but also to reasonably cut the operation and maintenance expenses as much as possible, while keeping both power quality and reliability requirements met. Reliability-centred maintenance (RCM) has been proven to be in response to this dilemma in power systems and has been yet successfully applied in various engineering contexts. This study introduces a novel approach, as of the first steps of RCM implementation in composite power generation and transmission systems, to identify the critical components for the main sake of a more focused maintenance management. Criticality evaluation is, here, concerned with components outage occurrence possibilities and cost-based consequences. A realistic market model has been implemented to account for the components outage consequences to the system different participants, that is, generation companies and distribution companies. The presented methodology is able to efficiently recognise the individual importance and contribution of each component in the cases of not only first order but also higher order contingencies. For the sake of demonstration, the proposed approach is applied to the IEEE reliability test system and IEEE 118-Bus test system, and the obtained results are discussed in detail